In a large nonstick skillet, heat 1 tablespoon of the olive oil over medium-high heat. Add onion, peppers and garlic. Cook for 7 minutes, stirring occasionally, so that garlic does not burn.
Add olives, stewed tomatoes, lemon juice, oregano, 1/2 teaspoon of the salt and 1/4 teaspoon of the black pepper. Break up tomatoes with a wooden spoon. Bring to a simmer.
Add the fish to the simmering sauce and tuck fish in so that it is covered with sauce. Simmer, uncovered, for 3 minutes. Turn fish gently; cook 3 more minutes, until fish is cooked through and flakes easily.
While fish is cooking, heat the remaining 1 tablespoon olive oil in a large saucepan. Add spinach and remaining salt and pepper. Cook until just wilted, about 2 minutes.
Serve fish with wilted spinach topped with the avocado slices. Makes 6 servings.