Place peas in a medium pot; cover with 6 cups water; let soak for 3 hours or overnight.
Bring water with peas to a boil (do not drain); boil for 15 minutes. Reduce heat to medium; gently simmer, adding water by 1/2-cupfuls as needed to keep peas covered, until peas are tender but still hold their shape, 20-25 minutes. Remove from heat and stir in 1 teaspoon salt. Season to taste with more salt, if desired. DO AHEAD: Can be made 1 day ahead. Cover and chill. Rewarm before continuing, adding more water by 1/4-cupfuls if too thick.
Place garlic on a cutting board. Sprinkle with salt and chop, occasionally smearing with the back of a knife, until a coarse paste forms. Tie thyme and bay leaf with kitchen twine to form a bundle for bouquet garni.
Heat 1/4 cup oil in a medium pot over medium heat; add garlic paste, bouquet garni, celery, onion, and bell pepper; cook until vegetables are tender, about 15 minutes. Add peas with their cooking liquid; bring to a simmer. Cook until flavors meld and sauce is thickened, 10-15 minutes. Remove from heat; stir in butter. Set aside.
Toast coriander seeds in a small dry skillet over medium heat, shaking pan often, until fragrant and slightly darker in color, 2-3 minutes. Let cool. Coarsely grind in a spice mill, or place in a resealable plastic bag and crush with a rolling pin or the bottom of a heavy skillet. Combine celery leaves, parsley, chives, lemon zest, and crushed coriander seeds in a small bowl; drizzle with oil and toss to coat. Season to taste with salt and cracked pepper. Divide peas among plates or transfer to a serving bowl; garnish with salad.