Preheat oven to 350°F. Lightly oil a 9 x 5-inch loaf pan.
Measure the 1 cup shredded part-skim mozzarella and set aside 1/2 cup for topping; use the remaining 1/2 cup in the batter.
Prepare produce: chop the 2 medium tomatoes and stem, seed, and dice the yellow bell pepper.
In a large bowl, whisk together 1/2 cup mozzarella, 6 egg whites, 6 whole eggs, 1/4 cup low-fat milk, 1/2 teaspoon baking soda, 2 tablespoons fresh thyme leaves, 1/4 teaspoon kosher or sea salt, and 1/4 teaspoon black pepper until combined and smooth.
Gently fold the chopped tomatoes and diced bell pepper into the egg mixture.
Pour the mixture into the prepared loaf pan and evenly sprinkle the reserved 1/2 cup mozzarella over the top.
Bake for 25 minutes, or until set — a toothpick inserted in the center should come out clean.
Remove from oven and let the loaf cool in the pan for about 5–10 minutes, then slice and serve.