Preheat your oven to 400F.

Place sliced squash in a large bowl. Toss with 1 teaspoon salt and set aside to let the salt pull the moisture from the squash for at least 10 minutes.

In a large sauté pan, heat butter over medium heat. Add diced onions and sauté until golden brown and caramelized, 8-10 minutes.

Remove the onions from the heat and mix in flour and seasonings until well mixed.

Return the mixture to the heat. Add milk and bring to a simmer. Reduce the heat to medium low and stir constantly until the mixture has thickened, 3-5 min. Remove the sauce from the heat, and remove the bay leaf.

Spread half of the sauce in a 9x9 baking dish. Rinse the salt from the squash slices and layer squash and tomato slices over the sauce. Spread the remaining sauce over top of the veggie slices. Top with the rolled oats.

Cover the dish tightly with aluminum foil and bake for 30 minutes, until the dish begins to bubble near the center. Remove the foil and bake for an additional 10 minutes, until the dish is fully bubbly all the way to the center.

Remove from the oven and let stand 5 minutes before serving.