Preheat the oven to 350°F (175°C). Gather all the ingredients: yellow squash, onion, cheeses, garlic, and crackers. prep

In a large skillet, melt the salted butter over medium heat. Add the yellow onion and ¼ teaspoon salt. Sauté for about 6 minutes, until the onion becomes tender and begins to caramelize. prep

Add the sliced yellow squash to the skillet and cook for about 5 minutes, until tender but still al dente. prep

Add the minced garlic to the skillet and cook for another 2 minutes, until fragrant. prep

Transfer the cooked onion and squash mixture to a 3-quart casserole dish. assemble

In a separate bowl, beat the eggs and add milk, fresh parsley, remaining salt, black pepper, and crushed red pepper flakes. Pour this egg mixture over the squash in the casserole dish. mix

In a bowl, combine half of the crushed crackers with the shredded cheddar cheese. Stir to combine, then add this mixture to the casserole dish with the squash. mix

Top the casserole with the remaining crushed crackers and sprinkle with Parmesan cheese. assemble

Bake the casserole in the preheated oven for 45 minutes, or until the top is golden and bubbly. bake