Spiralized Winter Vegetable Gratin

Ingredients

• Nonstick spray
• 3 large parsnips, peeled and cut into noodles with a spiralizer
• 2 medium sweet potatoes, peeled and cut into noodles with a spiralizer
• 1 small butternut squash, peeled and cut into noodles with a spiralizer
• 2 tablespoons extra-virgin olive oil
• Kosher salt and freshly ground black pepper
• 2 tablespoons unsalted butter
• 1 sweet onion, diced
• 2 garlic cloves, minced
• ¼ cup all-purpose flour
• 1 cup whole milk
• ¼ cup grated Parmesan cheese
• 1¼ cup grated Gruyère cheese, divided
• 2 teaspoons chopped fresh thyme
• 2 teaspoons chopped fresh rosemary
• 1 tablespoon chopped parsley