Look for sweet potatoes that are small in diameter, long, and uniform in shape.
Going vegan? Swap out the butter in step 3 with olive oil.
2 Tbsp olive oil
4 cups sliced onions (1/8 to 1/4-inch slices)
1 teaspoon fresh thyme, minced
1/2 teaspoon salt
Sweet Potatoes and Yukon Golds:
2 pounds of sweet potatoes, mixed in color if available, peeled and sliced into 1/8-inch thick rounds
2 pounds Yukon Gold potatoes, peeled, and sliced into 1/8-inch thick rounds
2 Tbsp melted butter
1 teaspoon of salt
1/4 teaspoon black pepper
1 Caramelize the onions: Heat olive oil in a large, thick-bottomed pot on medium heat. Add the sliced onions, toss to coat, lower the heat to medium low and cover. Let cook for 15 to 20 while you are slicing the sweet potatoes and yukon golds, stirring occasionally, until completely softened.
Then increase the heat to medium high and cook uncovered until golden brown. Remove from heat and stir in salt and thyme.
2 Peel and slice all of the sweet potatoes and yukon golds. Try to make them uniform in size.
Place the slices in a bowl of water while you are prepping them to prevent them for discoloring. If using purple sweet potatoes, keep the slices in a different bowl of water (the color bleeds).
Drain the slices, place them in a bowl.
3 Line baking dish with onions, sweet potato and potato slices: Place the caramelized onions in an even layer over the bottom of a 9×12 rectangular or oval casserole dish.
Arrange slices of sweet potatoes and Yukon Gold potatoes in rows on top of the caramelized onions.
Brush with melted butter, sprinkle with salt and pepper.
4 Cover with foil and bake in a 400°F oven for 45 minutes, until the potatoes are cooked through.
5 Remove foil, bake uncovered for 15 minutes more, until the edges are crispy and browned.
Sprinkle with a little more salt and pepper. Garnish with sprigs of fresh thyme. Serve immediately.