Preheat your oven to 350° F.
While your oven is heating up, dice your sweet potatoes and toss them in 1 tsp olive oil and some salt.
Roast your sweet potatoes for about 30 minutes, stirring about halfway through to promote even cooking.
While your sweet potatoes are cooking, in a large bowl, combine the eggs, almond milk, chopped rosemary, salt and pepper. Set aside.
Chop your onion. Heat a sauté pan with olive oil over medium heat, and cook until translucent. Set aside and allow it to cool. When cooled, add them the bowl with eggs.
Once your sweet potatoes are cooked, allow them to cool for a bit.
Line a pan with some parchment paper. Add the sweet potatoes to the bottom of the pan.
Pour the egg mixture over the sweet potatoes and bake in the oven for about 60 minutes. Casserole is completely cooked when a fork or toothpick is inserted into the middle and when removed is clean.
This casserole freezes well and can be heated up easily in the microwave.