Heat oven to 425ºF (218ºC) with racks set in upper-third and lower-third of oven.
In a small bowl mix together 1 tablespoon olive oil, 1 tablespoon maple syrup, and ½ teaspoon each of garlic powder, onion powder, smoked paprika, kosher salt and black pepper.
Working with one quarter of a squash at a time, place squash cut side up on a cutting board, and carefully cut ⅛-inch-thick slices crosswise down the squash, about 80% of the way through, stopping before you cut through the bottom. The squash piece should stay in tact but look accordioned. Repeat with remaining quarters.
Arrange squash on a baking sheet, cut side up, and brush evenly with spice paste. Transfer to lower-middle rack in oven and roast until tender, 15–20 minutes.
Meanwhile, melt 3 tablespoons butter in a large skillet over medium-low heat. Add ⅓ cup pepitas and cook, stirring frequently, until butter and pepitas are golden brown and fragrant, 4–5 minutes. Immediately remove from heat and stir in ½ teaspoon garlic powder and 1 tablespoon maple syrup, stir to combine. Then stir in 1 tablespoon apple cider vinegar. Let cool slightly then season to taste with salt; set aside.
Once squash is tender, turn oven to broil with rack set 6 inches from heating element. Broil squash until browned, 5-7 minutes. Transfer squash to a platter.
On one side of now-empty sheet pan, add halloumi; drizzle lightly with olive oil and sprinkle with ½ teaspoon dried rosemary. Add torn bread on other half and toss with 1 teaspoon olive oil to coat. Transfer to upper-middle rack in oven and broil until bread is lightly toasted and halloumi is browned. This will take about 5 minutes, stir bread halfway through to evenly toast.
Transfer cheese to a cutting board and cut into ¾-inch chunks.
Arrange squash on a platter. Top with cheese and toasted bread. Spoon browned butter-pepita sauce over top then sprinkle ¼ cup parsley over top and finish with a sprinkling of ground sumac (I use ½–1 teaspoon). Serve immediately.