I learnt in school! Maybe it was the area I grew up in, but we had a surprising amount of cookery lessons in primary school and in high school. I remember carrying the meals I'd created back on the bus each week, and we ate them as a family.
Since then, I found simple books useful. There was one that was a three or four ingredient cookbook that I liked. This is the link:
www.amazon.co.uk/Best-Ever-Three-Ingredient-Cookbook/dp/1844777790/ref=sr_1_3?s=books&keywords=three+ingredient+cook+book&tag=mumsnetforum-21&ie=UTF8&qid=1543779463&sr=1-3
I liked it because I hated recipes where it listed off 20 things you haven't got in, you end up not being able to do the recipe. This book (above) meant you only had to buy a few things, and the recipes were simple but nutritious. However I'm not sure how many of those are vegetarian, so it may not be an appropriate choice. I found it useful for me though.
Do you like stir fries OP? They can be a good place to start. You can just chop up some veg, then put it in a pan, with noodles, maybe some quorn (not in that order)... soy sauce. They are quite simple yet tasty. There's so much information online now, but I'd say get a basic book, and focus on easy things first. Maybe your Delia book is the best place to start?
For tomato sauce I'd look up recipes online.
I've never had a formal lesson outside of school, so I'd say I'm self taught and the things I cook are healthy.