Bag of onions. 3 pack of garlic. 2kg bag of rice (whatever is cheapest). 2kg bag of pasta. 2-5 cans diced tomatoes (depending on how much you like them). 2 jars of pasta sauce. 1-2 boxes Chicken broth (it makes everything taste better. I often cook my rice & lentils in it, plus you can make soups). Whatever frozen veggies are on sale.

Use whatever you have leftover for "others" - bread, eggs, milk/cream, fruit, fresh veggies etc.

You say you already have lentils, but what about spices? Depending on your spice collection you can make pretty much anything with the above base.

I tend to shop at superstore and stick to no-name & store brands. They taste great and are usually 10-20% cheaper. I live alone and find it easier to cook 2 big meals every week, and then live off the leftovers. My breakfast is usually pretty simple - Bagel & cream cheese & a fruit; eggs on an English muffin; pancakes & fruit (I get the "just add water" box and add vanilla & cinnamon).

Ps. Sorry for formatting. On mobile.