Heat oven to 400 degrees. Bake sweet potatoes until tender, about 45 minutes.
Heat oil in a large skillet over medium heat. Add onion and cook until tender, about 6 minutes. Add chard and cook, stirring, until bright green and wilted, about 5 minutes. Season with salt.
To serve, split potatoes and top each with the greens and 1/2 sliced avocado. Season with cayenne, salt, and a squeeze of lemon.
Refrigerate potatoes and greens in an airtight container up to two days.