Place tomatoes on a large rimmed baking sheet. Bake at 450° for 20 minutes. Turn tomatoes; bake 20 more minutes or until skins are blistered and lightly browned. Pulse tomatoes and pan juices in a food processor until roughly chopped.
Sauté onion in 2 tablespoons hot oil in a large skillet over medium-high heat 3 minutes or until tender. Add garlic and serrano; sauté 15 seconds. Add olives, capers, and chopped tomatoes; bring to a boil. Reduce heat, and simmer 5 minutes. Remove from heat; stir in oregano.
Sprinkle halibut with salt and pepper. Melt butter with remaining 2 tablespoons oil in a large skillet over medium-high heat. Add halibut; cook 5 minutes on each side or until fish flakes easily with a fork. Serve with sauce.