Walnuts and nutty-tasting quinoa are the stars in this vegetarian main. Choose short, squat bell peppers that will stand upright easily.
Preheat oven to 450 degrees, with rack in upper third. Slice a very thin layer from the base of a pepper so it sits upright. Slice off top, just below stem, and remove ribs and seeds from top and bottom. Repeat with remaining peppers. Discard stems; chop tops, and set aside.
In a medium saucepan, heat oil over medium. Add onion, garlic, coriander, and chopped tops; season with salt and pepper. Cook, stirring occasionally, until onion has softened, 4 to 5 minutes.
Add quinoa, and cook until fragrant, 1 minute. Add 1 cup water, and bring to a boil. Reduce to a simmer, cover, and cook until water has been absorbed and quinoa is tender, 11 to 13 minutes. Remove from heat, and stir in parsley, walnuts, and 3/4 cup provolone; season with salt and pepper.
Dividing evenly, stuff peppers with quinoa mixture; place in a 2-quart baking dish. Cover with aluminum foil, and bake until peppers are tender, about 1 hour. Uncover, and top with remaining 1/4 cup provolone; bake until cheese melts, 10 to 15 minutes more.