Pour beans into colander. Carefully pick over, discarding any broken ones. Rinse under running water. Discard any beans without skins or that float to surface. Drain well.
Heat a 6-quart pressure cooker on medium-high heat. Add 3 tablespoons olive oil. When oil is hot, add onions and ham hock and cook until the onions are wilted and the ham hock is nicely browned, about 5 to 7 minutes. Add minced garlic and bay leaf; cook for 1 minute. Add the beans and stir all ingredients together. Add water, remaining tablespoon olive oil, red pepper flakes, 1 tablespoon salt, and pepper (3 to 4 grindings or to taste). Stir again to mix well.
Following the manufacturer's directions, cover, lock the lid, and bring up to high pressure. Then, lower heat to maintain pressure and start timing. Cook at high pressure for 25 minutes.
Remove pressure cooker from heat and let rest for 5 minutes. Release the steam naturally, following the manufacturer's directions.
Once the steam is completely released, remove lid and taste the beans. If the beans are underdone, simmer them in the pressure cooker (without lid) until tender.
Remove and discard bay leaf. Remove ham hock and pick meat from the bone. Add meat to beans and discard the bone. Taste and adjust seasoning. Garnish with sour cream, grated cheese or thinly sliced scallions. Serve hot. Makes 4 to 6 servings.