1. Roast the bell peppers on a gas stove's open flame, turning them with tongs, until the skins are blackened, for about 2-3 minutes; or roast them on a rack under the broiler 2 inches from the heat, turning them every 5 minutes, for about 15 minutes. Transfer peppers to a bowl, cover with plastic wrap, and let them cool. When cool enough to handle, peel peppers starting at the stem end, carefully cut off the tops with a paring knife, and rinse out any seeds. Set aside.
2. Lightly coat a sauté pan with olive oil; place over medium heat. Sauté garlic until it begins to turn golden, about 1 minute. Add spinach; cook over medium heat until it wilts, about 2 minutes. Remove pan from heat. Stir in lemon juice and salt; place spinach in a bowl. Preheat oven to 350°.
3. In a small pot, cook couscous according to package directions (about 5 minutes). Using a spatula, add the couscous and the feta cheese to the wilted spinach, and mix well.
4. Line a baking sheet with aluminum foil. Generously stuff all the peppers with couscous-spinach stuffing, and place on a baking sheet. Bake on the center rack for about 8 minutes. Serve immediately.