1) Poke potatoes all over with a fork and lay them out on a baking sheet. Bake the potatoes at 350 degrees Fahrenheit for about an hour until they are tender.
2) Meanwhile, chop or rip chard off of the thick stems. Chop it roughly and rinse it very well in a colander. Rinse really well with cold water as chard will accumulate dirt in all the tiny creases in the leaves.
3) Slice an onion thinly.
4) Add olive oil to a large skillet over medium-high heat. Add onions and cook until they start to soften, but not brown, about 3-4 minutes. Season with a pinch of salt and pepper.
5) Add chard to pan and continue to stir. Chard should cook down and reduce substantially after 5-6 minutes. Season mixture with salt and pepper and turn heat down to low. Keep mixture warm until needed.
6) When potatoes are tender, remove from oven and split in half. Lightly mash each half with a fork.
7) Divide filling between the four baked potatoes. Top with 1/4 avocado per potato. Sprinkle each potato with Kosher salt and cayenne pepper. Serve immediately with a wedge of lemon.