Serious Eats Food Lab [1] Egullet [2]

I can also recommend Modernist Cuisine, which is a sort of text book version of On Food and Cooking. Though it has a lot of industrial equipment and ingredients that aren't applicable to home cooks, it has excellent photos and diagrams that illustrate the cooking process from a biological and physical level. (Modernist Cuisine at Home isn't nearly as comprehensive, though it shirks a lot of the weird/expensive equipment and additives.)

Also Modernist Bread for breads is fantastic and comprehensive.

1. https://www.seriouseats.com/the-food-lab

2. https://forums.egullet.org/

Nathan Myhrvold and co: https://modernistcuisine.com

Dave Arnold: http://www.cookingissues.com http://heritageradionetwork.org/series/cooking-issues/

Salt Fat Acid Heat is another great cooking theory textbook. Amazing writing and the lessons can apply to any meal you'll ever make.