Step 7. Bake for approximately 25 to 45 minutes at 350 degrees Fahrenheit. Cooking time depends on the oven temperature, quantity and size of steaks. For example, a 7-ounce tuna steak bakes in 20 minutes at 400 degrees Fahrenheit.tep 1. Preheat the oven to between 350 and 450 degrees Fahrenheit. You can bake tuna at any temperature within this range, but cooking times will vary according to temperature, size and thickness of the tuna steak.