Soak the peas: Place the peas in a large bowl. Add enough water to cover the peas by about an inch. Cover the bowl and let the peas soak overnight (at least 8 hours). Drain the water from the peas before cooking.
Cook the bacon: In a large Dutch oven or heavy-bottomed pot, cook bacon over medium heat until crisp. Use a slotted spoon to remove the bacon to a paper towel-lined plate. Reserve the drippings in the pot.
Sauté the aromatics: With the pot over medium heat, add the onion and celery to the drippings and cook until the onion is translucent, about 5 to 7 minutes. Add the garlic and cook until fragrant, about 30 seconds.
Simmer the peas: Stir in the ham hock, 5 cups of water, drained peas, thyme sprigs, bay leaf, salt, and pepper. Bring to a boil, then reduce the heat so that the liquid is at a gentle simmer. Cook, uncovered, for 1 hour, stirring occasionally.
Add the rice: Stir in the rice, cover the pot, and simmer gently for 20 more minutes, or until the rice is done and the peas are soft. Check on the rice toward the end of the cooking time and add a little more warm water if necessary. By the end, most of the water should be absorbed, but the mixture will still be moist.
Rest: Still covered, remove the pot from the heat and let the rice and peas rest for 10 minutes.
Finish and serve: Remove the cover, discard the ham hock, bay leaf, and thyme sprigs, and fluff the rice gently with a fork. Taste and season with additional salt, pepper, and red pepper flakes if desired. Add a dash of apple cider vinegar (optional). Garnish with crispy bacon, sliced green onions, or fresh herbs, and serve!