Peel and slice 1 onion. Core 2 peppers and then thinly slice.
In a large cast-iron skillet, heat 2 T. olive oil and add the onions. Saute until they are tender, then add the peppers along with 1 minced clove of garlic. Sprinkle with salt and pepper. Cook for another 5 minutes, then remove from heat.
Trim 1/2 lb. of green beans and cut into 2-3 pieces.
Cut 2 Delicata squash lengthwise, then scoop out the seeds and pulp. Slice the squash halves crosswise.
Layer the squash and beans on top of the onion/pepper mixture in the pan, using the beans to fill the gaps between the squash slices. Sprinkle with 1/2 t. dried thyme plus salt and pepper.
Top the casserole with thinly sliced tomato rounds, then cover them with 1 C. grated gruyere cheese.
Bake at 350 until the gratin is bubbling and the cheese is nicely browned.