Preheat oven to broil and position rack under the broiler.
Once the skillet is hot, add oil and swirl to coat the bottom.
2 tbsp extra virgin olive oil
Add onion and garlic to the skillet and cook, stirring occasionally, until softened, 3-4 minutes.
Trim ends from the zucchini and grate coarsely.
Add zucchini and salt to the skillet; cook, stirring occasionally, until zucchini is tender and most of the liquid has evaporated, 5-6 minutes. Use a spoon to remove excess liquid if necessary.
1 tsp salt
Meanwhile, grate mozzarella. Whisk eggs in a large bowl; add half the cheese and stir to combine.
1 (8 oz) block mozzarella cheese
12 eggs
Spread the zucchini evenly in the skillet. Reduce heat to medium-low and pour in the egg mixture. As eggs set around the edge of the skillet, gently lift cooked portions with a spatula to allow uncooked egg to flow underneath. Cook until bottom and sides are firm but the top is still runny, 5-7 minutes.
Slice tomatoes into wedges and place in a medium salad bowl; drizzle with olive oil and season with salt and pepper.
2 tsp extra virgin olive oil
¼ tsp salt
⅛ tsp black pepper
Sprinkle remaining cheese over the frittata; place skillet in the oven and broil until the top is set, 2-3 minutes.
Pick leaves off the stems of the basil, roll up crosswise, and thinly slice into ribbons. Add to the bowl with the tomatoes and toss to combine.
To serve, cut frittata into wedges and divide between plates with the tomato salad on the side. Enjoy!