Lentils…

I add lentils to shepherds pie & bolognaise.
I use tinned green lentils and no one ever knows they are there.

I always add a handful of red lentils to mince, whether for cottage pie or e.g. spag Bol. I add them with stock/water (and sundry finely chopped veg) once I’ve browned the mince and onions, and then simmer on low for around an hour. By then they’re virtually broken down and you can barely tell they’re even there.

I have to say I'm not a fan of the texture of lentils.
In a beef mince cottage pie I add a tin of ordinary baked beans.
Would plain cannellini beans work with lamb?

Red lentils with vegetables and whatever meat or sausage or bacon or nothing, cooked slowly in water make a wonderful stew/soup, I do it a lot. No need to pre soak the red lentils just add them to the mix. Cook slowly stir regularly and enjoy,
The lentils thicken the soup and are invisible . Healthy and very easy.

We do the swede potato combination, it works well,

I use the red ones often, usually in the slow cooker, where they break down to hardly being noticed.
Have you tried tinned Mamma if you are short of time?

Brown lentils are best here I think.
Just cook them separately first till they are nearly done but still a little on the firm side.

I wouldn't use red ones or they will go porridge-y. Even whole red ones are less firm than whole brown (or green).
Personally I don't know about tinned ones. Aren't they already fully cooked?

I get these sachets from Waitrose from time to time - very tasty

I always add a handful of red lentils to mince, whether for cottage pie or e.g. spag Bol

Same.
Personally, I put them in either near the beginning or half way through cooking. Personally I cannot taste them[I dont put in huge amounts}. Should add, I am no cook.

When I cook my soups with red lentils always cook for 90 mins . Your just boil up some but instead of just water add Oxo cube. Once cooked and drained you could blitz them with a hand blender and bit of cooking liquid then add to the lamb mixture and he wouldn't know they where there. Have you tried using Quorn mince for a cottage pie if you add meat Oxo he would know it wasn't meat.
My brother loves meat but when I made a Quorn sausage casserole I had browned the sausages first and cut each one into 8 with all the veg and pearl barely he didn't know it wasn't meat. He had seconds . Only later I told him what it was and he didn't notice the difference.

I make dhal with red lentils and onions. It is quite porridgy and it should be. Lentils require a lot of cooking to make them digestible.
So tinned ones would definitely be an option for you.

I just buy mine in tins from Waitrose, less faff.
However, Red lentils I only uses in soups or stew where they have plenty of time to cook.

Red lentils Cook quickly when added dry, straight into the liquid, I cook a huge pot of soup in about 40 mi utes. The lentils disappear and together with potatoes and parsnips, do their job of thickening.

I also only use the red ones in soups as the mouth-feel is somewhat drier than the others. If they are going in with mince I would choose others.

Exactly Silverlining

I make dhal alot I love it and refried beans I use lentils beans and grains in many dishes. It reduces the amount of meat in a meal and adds interest.

Witzend

I always add a handful of red lentils to mince, whether for cottage pie or e.g. spag Bol. I add them with stock/water (and sundry finely chopped veg) once I’ve browned the mince and onions, and then simmer on low for around an hour. By then they’re virtually broken down and you can barely tell they’re even there.

My DD can sense a lentil from 100 yards! She really dislikes them.

I think if you don't tell him, only use a few, then wait to see if he notices or just says "that was lovely" it's worth a try again, Mamma66

If not, at least there's vegetables in the pie which you could increase.

I cook finely diced onions until translucent. Add lentils, stir, add broth, cook until soft, add with veg into shepherd's pie.

We eat lentils most every day, in some meal. Cheap protein.

I use put lentils.
Simmer them for around
20-25 minutes, then fry them in with the other ingredients.

Puy lentils!!

Lentils aren’t great for Diabetics. Just check his levels two hours after he’s eaten. Also, try celeriac instead of potatoes. Also carrots are great, but I find I can tolerate a small amount.

I grate cheese on top before putting in the,oven which makes it all yummy. I also add a tin of tomatoes to the mince.

Lentils cook quicker in a pressure cooker - about 10 minutes.

I put lentils, the dried orange coloured ones, in every mince dish. I stir them in just before I add any liquids, and then cook in the slow cooker for 2 hours, they cook to softness and just melt into the dish.