Step 1

Preheat oven to 400F.

Step 2

In a heavy bottomed skillet, melt 4 tablespoons butter over medium heat. Add red onions and cook for about 2 minutes, until they start to soften. Add squash and ½ teaspoon salt. Cook until squash starts to brown, about 5 minutes. Pour in stock, bring to a simmer, reduce heat and cook for about 5 more minutes.

Step 3

Meanwhile, make the sauce. In a small saucepan, melt remaining 2 tablespoons of butter. Remove from heat and whisk in flour until a thick paste forms. Slowly add milk, whisking constantly. Return saucepan to heat and continue to whisk until mixture starts to thicken, about 3 minutes. Remove from heat and stir in 1 teaspoon salt, Dijon mustard, and half of the cheese.

Step 4

Transfer squash and onions to a 9 x 13 inch baking dish. Add tomatoes, tucking them all around the squash and between layers, so their evenly spread throughout the baking dish. Pour sauce on top and use a wooden spoon to mix throughout. Top gratin with remaining cheese and parsley.

Step 5

Bake for 15 minutes until squash is soft and cooked through. Turn broiler on high and finish under the broiler for 5 minutes, so cheese starts to brown. Let cool for 10 minutes before serving.