Tinned tomatoes
Tomato puree
garlic
onions
olive oil
red wine (I don't drink so buy smaller bottles for cooking)
Worcestershire sauce
stock cubes.
salt and pepper
With the above I use as a base for things like -
-bolognese (add Italian herbs, mince, tonnes of chopped veg like peppers, courgette, mushrooms, carrots)
-chilli (mixed beans/kidney, black, cannellini, chili flakes and smoked paprika)
-casserole (add smoked paprika, tin of any beans, bacon, chicken or sausages, mushrooms, carrots)
-cottage pie (mince or lentils, carrots, potatoes)
-curries (curry powder or cumin/ginger/chilli and choice of meat and veg/pulses - chickpeas are a good filler as they hold their shape well)
Cornflour is also helpful for thickening sauces (though not essential)
White wine for non-tomato based casseroles
Simple white sauce= butter, flour and milk (if you're not a milk drinker, we always have long life milk in the cupboard which is perfect for white sauce).
Use with bolognese for lasagne, add cheese for a cheese sauce to have with macaroni, pour over sliced layered potatoes/onions/bacons for an oven bake (serve with greens), use as a pasta bake with vegetables/meat of choice, base for a pie filling with leeks/bacon/chicken/mushrooms.
Flour/milk/eggs also used for pancakes (delicious pumpkin pancake recipe in one of the Hiary Dieters books. I will roast a butter squash and mash then put some in freezer if I don't need it all, but the pancakes also last well in freezer - add sugar and cinnamon to the list!)
We always have hard cheese, bacon, eggs in the fridge (spaghetti carbonara, omelettes) and Greek yoghurt for healthy breakfasts/snacks with puffed wheat/oats/raisins/nuts/tinned fruit like peaches and honey
Different types of pasta, wraps, rice, noodles always in the cupboard.
Sometimes I make my own flatbreads as well but can't always be bothered (flour, salt, honey, water, instant yeast). Nice with chilli honey halloumi and home made tzatziki (see below)
We rarely get frozen chips - homemade wedges instead: salt, potatoes, olive oil, smoked paprika.
I generally don't make my own sauces with the exception of tzatziki to have with wraps greek yoghurt, cucumber, lemon juice, garlic, seasoning.
The Hairy Bikers have some great fakeaway recipes in their Dieters books. The Sweet & sour chicken, Chicken Bhuna and Korma are some of our faves. We always buy a tonne of chicken and then do big batches to freeze.
Homemade pizza is very simple (but I only make on a non-working day so I can prep the dough earlier in the day) flour, yeast, salt and water for base, tinned tomatoes/tomato puree/Italian herbs for sauce plus your topping/cheese.
DH and I used to eat so much processed food before we had the kids but the last 10-15 years have really transformed how and what we cook so the majority of our meals are made from scratch. Batch cooking means we always have something tucked away for busy or stressful days. I also usually squirrel stuff away in smaller containers for my lunches (before the DC get a chance for seconds!).
We have a very small kitchen and a regular upright freezer (counter height so 4 draws, one of which is always filled with bread - the one upfront I really haven't been blessed to shift from our house, though I don't eat it much - the kids do though for sandwiches). Our oven is also very small. We did invest in an extra fridge which we mainly use for milk and yoghurt - we get through a lot and our weekly shop would fill our main fridge.
I'd invest in some containers for freezing if you're going to start batch cooking (which is definitely worth doing - saves a lot of time and effort).
Good luck!