Strip collard leaves from stems and wash greens. Bring a few quarts of water to a boil. Add salt and greens, then simmer for 10 minutes. Scoop greens into a bowl. Add potatoes to cooking water and simmer until tender, 7 to 10 minutes.
Meanwhile, in a large skillet over medium heat, cook bacon until browned. Transfer bacon to paper towels to drain. Discard fat and wipe out pan. Return pan to medium-high heat, add oil and, when it's hot, add onion. Cook for 5 minutes.
Coarsely chop cooked collards, then add them to pan along with garlic and pepper flakes. Scoop some of the potato water into the pan so everything cooks in a little moisture, adding more water as needed. When potatoes are tender, scoop them out and add them to greens. Add bacon and toss everything together. Taste and season with salt and pepper. Keep everything distinct, or mash potatoes into greens. The latter is especially good, but messy looking. Transfer to a platter, season with hot pepper sauce to taste and serve.