Two-Potato Gratin

INGREDIENTS

1/2

sweet yellow onion (diced)

- tablespoon butter

- tablespoon olive oil

1 3/4 cups

heavy cream

2 tbsp

chopped parsley

- tablespoon fresh chives (chopped or 1 teaspoon dried chives)

1/2 tsp

fresh rosemary (chopped or ¼ teaspoon dried rosemary, chopped)

1/2 tsp

ground black or white pepper

1/4 tsp

Kosher salt

1/8 tsp

freshly grated nutmeg or dried ground nutmeg

1 1/2 lb

4-5 medium Yukon Gold potatoes, peeled & thinly sliced

1 1/2 lb

3-4 sweet potatoes, peeled & thinly sliced

2 cups

milks – any type of milk is fine (it will be discarded)

1 1/2 cups

Gruyere Cheese (high-quality), shredded

1/2 cup

grated Parmesan cheese