Rinse and sort black-eyed peas. Place peas and bacon in a Dutch oven; add water to cover. Bring to a boil; boil 2 minutes. Remove from heat; let soak, covered, 1 hour. Do not drain.
In a cast-iron or other heavy skillet, heat butter over medium-high heat. Add onion; cook and stir until tender. Add garlic; cook 1 minute longer. Stir in thyme.
Stir into pea mixture. Bring to a boil; reduce heat. Simmer, covered, until peas are tender, stirring occasionally, 30-40 minutes. Sprinkle with salt to taste. If desired, garnish with fresh thyme leaves and additional crumbled bacon.