Grease a 1 1/2 quart baking dish (I used an 8X8 dish).
Melt the butter in a small skillet over medium-low heat.
When the butter is foamy, stir in the breadcrumbs.
Cook, stirring, until the crumbs are evenly crisp and golden brown.
Combine the squash, onion, and salt in a large saucepan.
Cover with water and bring to a boil. Reduce the heat to low and simmer until the vegetables are tender, 15 to 20 minutes.
Drain and mash well.
Stir in the milk, eggs, melted butter, flour, cheese, and thyme and add salt and pepper to taste.
Spoon into a baking dish and bake for about 30 minutes.
Top with buttered breadcrumbs and bake for another 10 minutes longer, until browned.
Notes
2 tbsp butter
1 cup breadcrumbs (I used panko)
2 lbs yellow crookneck squash, sliced
1 medium onion, chopped (I used 1/2 a Vidalia onion)
1 tsp salt
1 cup milk
2 large eggs, lightly beaten
3 tbsp butter, melted
3 tbsp flour
2 cups Cheddar cheese, shredded
2 tsp fresh thyme
salt and pepper, to taste