Preheat the oven to 400°F.

Melt the butter over medium heat in a cast iron pan or ovenproof pan. Add the leeks and lightly salt. Reduce the heat slightly and cover. Cook 5 to 8 minutes, stirring often, until the leeks soften and lightly brown. Add the garlic and cook until fragrant ~ about 1 minute. Nestle the squash and tomato slices into the leeks, evenly spreading them across the surface of the pan. Cook over medium for 5 minutes.

Meanwhile, whisk together the egg whites, lemon zest, basil, and a pinch of salt and pepper. Pour the mixture over the vegetables and turn off the heat. Using a spoon, scoop the goat cheese in pieces of about 1 teaspoon and spread evenly across the pan. Place the pan in the oven and bake for 20 to 25 minutes or until desired doneness (I prefer my eggs slightly more cooked, so I baked my frittata for 25 minutes ~ the eggs were firm in the middle when I touched them with my finger. If you like your eggs less firm, cook for 20 minutes and the center of the frittata should have a slightly jiggle).

Slice and plate. Sprinkle with fresh herbs, if desired. Pairs great with a side salad! Enjoy!