Preheat a grill for cooking over medium heat. Lightly oil the grill grates.
Melt the butter in a small saucepan and turn off the heat. Remove 1/4 cup of the melted butter and reserve. Whisk the maple syrup, cider vinegar, dry mustard, crushed red pepper, 3/4 teaspoon salt and a few grinds of pepper into the saucepan. Bring to a simmer and cook for about 4 minutes to reduce slightly. Remove the glaze from the heat and let cool. Set aside 1/3 cup of the glaze for drizzling at the end.
Carefully halve the squashes from the stem to the bottom. Scoop out the seeds with a spoon and discard. Place the squash halves flat-side down and slice crosswise into 1/2-inch scalloped half-moons. Place the slices in a very large bowl.
Mix together the chili powder, coriander, cinnamon, 1/2 teaspoon salt and 1/4 teaspoon pepper in a small bowl. Sprinkle the spice mixture over the squash slices. Drizzle the reserved 1/4 cup of melted butter over the squash. Toss with your hands to make sure the slices are evenly coated.
Place the squash slices on the grill and cover. Cook until dark grill marks appear, about 10 minutes. Flip the squash slices; cover and cook until the squash is just tender enough to be pierced easily with a paring knife, 7 to 8 minutes more.
Brush the squash slices with the glaze and flip. Cook 1 minute, brush with glaze, flip and cook on the other side for another minute. Remove from the grill and arrange on a large platter. Drizzle with the reserved glaze. Sprinkle with the pepitas. Serve warm or at room temperature.
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