The food is substantially cooked on board. I take the train to or from Vancouver once or twice a year (and fly the other on AC to keep this relevant). If you ask nicely the manager may let you have a peak in the kitchen. Most main courses are definitely cooked on board: prime rib, fillet steak, rack of lamb, fish, chicken, eggs. Some labour-intensive items are prepared at the caterers, such as lasagne and stuffed baked potatoes. Desserts are catered as are the dinner rolls. .If you want your steak or roast beef served "blue" or well-done, you just need to ask.

As others have pointed out, the food on the "Ocean" is reheated because there is only a galley for preparation not a real kitchen in the Renaissance equipment. Those meals are mostly surprisingly good, although I tend to avoid reheated red meat. Don't let anyone tell you they are micro-waved. They are not. There are convection ovens. The tiny microwave is used for heating rolls. Toast is toasted on board.