Heat the olive oil in a large pan on medium-high heat.
Sauté diced onion and red pepper with 3 cloves of chopped garlic until soft.
Thinly slice the zucchini, summer squash, and peeled sweet potato. Consider using a mandoline for slicing.
Make horizontal stacks of alternating vegetables (zucchini, sweet potato, summer squash) and place around a high-sided oven-safe sauté pan. Remove from the pan once you get a feel for how much you need to fill the rest of the bottom of the pan.
Add remaining slices of vegetables with rosemary, salt, and pepper to the onion/red pepper/garlic until completely soft.
Blend sautéed veggie mixture with a tablespoon of olive oil until smooth, and coat high-sided oven-safe sauté pan—save some for plating!
Place vegetable stacks around the pan starting on the outside, then on the inside.
Drizzle olive oil, salt, pepper on top of stacks.
Cover with parchment paper and roast at 300° for 90 minutes.