Preheat oven to 400° F
Coat pearl onions and sweet potatoes with olive oil and dust with salt and pepper.
Arrange in a single layer on a baking sheet and roast for 25 minutes. During this time you can prepare the veggies for the frittata.
Remove roasted onions and sweet potatoes from the oven and let cool to room temperature.
Preheat oven to 350° F.
In your largest skillet, over medium heat, lightly fry the sage leaves in olive oil. Remove to a paper towel lined plate and let cool.
In same skillet, adding a little olive oil if necessary, add the garlic and sautée until fragrant.
Add mushrooms. Sautée until they pass the point of giving off moisture and just start to stick to the pan a little.
Add swiss chard. Sautée until wilted. This doesn't take long. Remove skillet from heat.
Distribute cooked mushrooms and swiss chard on the bottom of your baking dish.
Pour egg and milk over the vegetables.
Crinkle the fried sage over the top, try to distribute evenly.
Sprinkle half the parmigiano reggiano on top.
Bake for 20 minutes.
Spread the other half of the cheese on top.
Bake until cooked through, about another 15 minutes. Finish assembling the salad during this time.
Let sit for 5 minutes. Serve warm with the salad.