sweet potato or yam?!? — Les Petites Gourmettes

Ingredients

• 1 of each; red sweet potato, tan sweet potato, russet potato, Yukon Gold potato, red-skinned potato and white-skinned potato (about 4 pounds total), each peeled and thinly sliced
• 1 of each; large red onion and large yellow onion, each peeled and thinly sliced
• 1 tablespoon salt
• 4 tablespoons (1/2 stick) unsalted butter, divided
• 2 ounces watercress leaves, coarsely chopped, stems discarded
• 1 cup fat-free half-and-half
• 1 cup heavy cream
• 2 garlic cloves, peeled and finely minced
• 1 cup shredded Fontina cheese