One of my favourite frugal 'hidden vegetable' meals is bolognaise/chilli. (Basically the same recipe).
400g decent beef mince, low fat if you can get it. Try checking for special offers or at the end of the day on the reduced section.
200g brown/green lentils, soaked in plenty of boiling water & drained.
1 courgette, coarsely grated
1 carrot, finely grated
About 8 large mushrooms, finely chopped, or sliced, as you prefer
2 medium onions, finely sliced
2 tins basic chopped tomatoes
1/2 beef stock cube
3 cloves garlic, crushed, or 2 tsp garlic puree
1 tbsp oil for frying
2 heaped tbsp mixed italian herbs & a bayleaf (if making bologaise)
1 tsp thyme & 2x 400g tins kidney beans (if making chilli)
Splash of red wine if you have any around. (ha!)
You can leave out or change the veg but they are there to bulk it out & this does go a lot further if you use them. If you do leave them out you need to reduce the herbs.
Fry onions in the oil in a big, lidded pan till starting to soften, then add the mince, make sure you break it up. When it's browned, stir in the garlic, courgette, carrot & mushrooms. Fry, stirring occasionally, till the liquid comes out of the mushrooms.
Add the tinned tomatoes, stock cube, herbs, pre-soaked & drained lentils and, if using, the drained & rinsed kidney beans and/or wine. Add enough water to cover. Stir in & leave to simmer, stirring occasionally to prevent sticking.
This will be ready when the excess liquid is mostly gone & the sauce is thickened. Check for seasoning at the end of cooking at the sauce can thicken a lot.
The bolognaise is good for pasta sauce, pizza topping, to go with baked potatoes. It freezes well and the quantities here will make several generous servings.