Preheat oven to 350 F.
Melt butter in a skillet/pan over medium heat. Add garlic and gradually add flour, constantly whisking at the same time until a roux has formed.
3 tablespoon butter, 2 large garlic cloves, 5 tablespoon all purpose flour
Stir in the veggie broth, milk slowly and whisk vigorously.
½ cup veggie or meat broth, 1 cup milk
Finally, add in the cheddar and whisk until the cheese is fully melted and set aside.
1 ½ cup sharp cheddar cheese
Layer the potato slices in an 8x12 or 9x13 inch dish (butter or spray with cooking spray prior), add the onion sliced in between. Sprinkle with salt, pepper, and dried thyme. 2 pounds starchy potatoes (I like Golden), ½ teaspoon dried thyme, 1 ½ teaspoon kosher salt, ¼ teaspoon ground black pepper, 1 medium brown or red onion
Pour the cheese sauce all over the potato slices, cover with foil and bake for 1 hour 15 minutes.
Remove the foil and top with mozzarella cheese and return to the oven until cheese is melted and bubbly, slightly browned. You can use the broiler method but be careful as the cheese can burn very quickly.
½ cup mozzarella cheese, fresh thyme sprigs, 1 teaspoon fresh parsley