Prepare: Heat olive oil in a large skillet over medium heat, then add onions and garlic and sauté until onions become translucent.
Add: Stir in diced tomatoes and Herbs de Provence, then add haddock fillets to the skillet and spoon some of the tomato mixture over the top.
Cook: Cover and simmer for 15-20 minutes, until haddock is cooked through and flaky.
Serve: Plate the haddock fillets and spoon extra tomato sauce over the top. Serve hot.