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.
Drain the water from the peas.
In a large Dutch oven or other heavy-bottomed pot, cook bacon until crisp. Use a slotted spoon to remove the bacon from the pot; place on a paper-towel lined plate. Reserve the drippings in the pot.
With the pot over medium heat, add the onion and celery to the drippings and cook until the onion is translucent, about 5-7 minutes. Add the garlic and cook until fragrant, about 30 seconds.
Stir in the ham hock, 5 cups of water, 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.
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 towards the end of the cooking time, and add a little bit more warm water, if necessary. By the end of the cooking time, most of the water should be absorbed, but the mixture will still be moist.
Still covered, remove the pot from the heat and let the rice and peas rest for 10 minutes.
Remove the cover, discard 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!