Combine first 4 ingredients in a bowl, stirring well with a whisk.
Heat oil in a large cast-iron or oven-proof skillet over high heat. Add onion; sauté 2 minutes. Add sage and garlic; sauté 1 minute. Add pepper and tomatoes, stirring well. Bring to a boil; cook 1 minute. Stir bread into tomato mixture; top with cheese. Pour egg mixture over tomato mixture (pan will be very full).
Carefully place pan in oven; bake at 450° for 25 minutes or until egg is set. Let stand 5 minutes. Cut into 4 wedges.