Summer Squash Gratin

4 tablespoons butter
3/4 cup panko breadcrumbs
1/2 cup grated Parmesan cheese
1/2 cup onion, chopped
2 cloves garlic, minced
2 pounds summer squash or zucchini or both, sliced
2 teaspoons fresh tarragon leaves or 1 teaspoon dried
1/2 teaspoon sea salt
Black pepper, to taste
1 cup heavy cream

1. Heat oven to to 400°F. Melt 2 tablespoons of the butter and mix with the panko and Parmesan in a bowl. Set aside.
2. Heat the remaining 2 tablespoons butter in a 10 inch cast iron skillet over medium heat. Add the onions and cook, stirring occasionally, about 3-4 minutes until softened. Add the garlic and cook, stirring, for 1 more minute.
3. Stir in the squash, tarragon, and salt. Season with pepper and cook for a few minutes until squash is tender crisp, a few minutes. Stir in the cream. Sprinkle the panko mixture over the squash.
4. Place skillet in oven and bake until the top is golden-brown, about 20-25 minutes. Let cool a few minutes before digging in.