Butternut Squash Sandwich with Cheddar Cheese and Pickled Red Onion

Name: Butternut Squash Sandwich with Cheddar Cheese and Pickled Red Onion
Ingredients: 1 medium butternut squash (about 2 pounds); 1/4 cup extra-virgin olive oil; 8 sprigs thyme; 2 garlic cloves, smashed; 1 3/4 teaspoons kosher salt, divided; 1/2 teaspoon freshly ground black pepper; 1/4 cup red wine vinegar; 1 1/2 teaspoons sugar; 1/2 medium red onion, thinly sliced; 1/4 cup mayonnaise; 2 tablespoons Dijon mustard; 4 ciabatta rolls; 4 slices white cheddar; 1 cup arugula; Chips (for serving; optional)