Wash 2 lbs. of sweet potatoes and cut into thin rounds, 1/8″ thick.
Thinly slice 2-3 summer squash the same way. You want about half as much squash as sweet potato.
Slice 2-3 spring onions, white and green parts. Cut the fatter slices in half. Set aside 1/4 C. Mix the rest of the onions in a bowl with 1 C. grated parmesan cheese, 2 T. flour, 1 T. fresh thyme leaves (or 1 t. dried), 1 1/2 t. salt and 3/4 t. pepper.
Preheat the oven to 375. Butter two 8″ cake pans (pie pans or baking dishes will also work). Make a layer of sweet potatoes, overlapping them slightly. Make a layer of squash. Drizzle with 1 T. olive oil and sprinkle with cheese mixture. Repeat. Then make a final layer just of sweet potatoes and top with cheese mixture.
Repeat for the other pan.
Cover the tortes with foil and bake for 30 minutes or until the sweet potatoes are almost tender. Uncover and bake for another 25 minutes, until the tortes begin to brown and the sweet potatoes are fully tender.
Cut into wedges and serve sprinkled with the remaining onions.