This is a simple and light savory gratin.
Preheat the oven to 375 degrees.
Clean 3-4 spring onions and cut the white parts in half lengthwise, then chop roughly. Reserve the greens.
Trim the greens off 1 fennel bulb, then thinly slice the bulb into rounds.
Toss the onions and fennel in a 9×12 baking dish with 2 T. olive oil plus salt and pepper.
With a heavy-duty vegetable peeler, peel medium 1 butternut squash. Cut in half and discard the pulp and seeds from the seed cavity. Place the squash cut-side down and slice into half rounds, about 1/2 inch thick.
Mince fresh sage or rosemary leaves to make 1 1/2 T. You can also use thyme. Or a small amount of all three.
Arrange the squash pieces in one layer on top of the onions and fennel. Sprinkle with salt, pepper and half the herb. Brush with olive oil. Repeat to make a second layer.
In a small bowl, mix together 1 C. panko or other breadcrumbs plus 3/4 C. gruyere cheese. Add the remaining herbs as well as more salt and pepper. Cover the gratin with a thin layer of the mixture.
Bake for 40 minutes or until the squash is tender. Raise the temperature and cook another 5-10 minutes, until the top is golden brown and crispy.
Mince the spring onion greens to make 1 C. and sprinkle over the top of the cooked gratin.