Melt the 1/4 cup butter in a medium cast iron skillet over medium-high heat. Add the onion and cook until softened, stirring occasionally; add the garlic and cook for 1 minute. Stir in the parsley, season with salt and pepper to taste, and remove the skillet from the heat.
Soak the bread in ice water to cover until softened, then squeeze out the moisture and chop. Add to the onion in the pan and cook over medium-high heat for 3 minutes, stirring occasionally. Add the squash, cook 3 minutes, stirring occasionally, and remove from the heat. Stir in the eggs and season with salt and pepper to taste.
Divide the mixture between the prepared mini-servers. Sprinkle each with the crushed crackers, then dot with the pieces of butter. Bake until hot and golden brown on top, 20 to 25 minutes.