Preheat oven to 375 degrees.
In a large, oven proof pan, heat olive oil until shimmery on medium high.
Add in diced onion, cook about 8 minutes, until soft and slightly browned.
Add diced tomatoes, herbs (if you aren't using herbed diced tomatoes add some basil and oregano- about 2 tsp each), and capers.
Sitr around and let reduce slightly, about five minutes.
Add fish to pan.
Pop in oven and bake about 15 minutes- until fish is flaky and white. This cooking time depends on the size of your filets and your oven- check after 8 minutes, and then again after 12.