There's a recipe from an Instagram reel that I love at the moment that would make use of the rice and lentils and inexpensive additional ingredients.
You add rice, stock and grated veg to an ovenproof casserole type bowl, cover and cookuntil water absorbed. Top with cheese sauce, or just cheese, and let it get bubbly. For your situation I'd buy a bag of carrots for around 60p, and use those with the rice and lentils (for a bit of protein for a nicely filling and balanced meal), a stock cube and whatever dried herbs you have that would go, and an onion although it would be fine without that. Grated cheese on top if you can.
The account is broccoli_mum if you want to go find the original recipe for quantities. She makes it vegan, but I've found it's really versatile to using different grains, different veg, and normal cow's milk cheese sauce or just cheese on top.
A second, not dissimilar recipe suggestion is from somebody in mumsnet for a carrot & cheese oat bake.
Carrot and cheese bake
Serves four
14oz of carrots grated ( or two large carrots per person
3oz of porridge oats
1oz of flour
A few tables spoons of vegetable water or stock
1 teaspoon of dried thyme (could omit)
4oz of cheddar cheese, grated
Salt and pepper seasoning
Mix thoroughly
Press into greased dish and bake for 40 minutes