Preheat the oven to 400 degrees F. Add the vegetable broth and lentils to a medium saucepan and place over medium-high heat. Bring to a boil, then reduce to low and cover. Cook until the lentils are tender, 30 minutes.
Meanwhile, heat 2 teaspoons of olive oil in a separate saucepan over medium heat. Add the onion and garlic and cook until the onion is translucent and tender, 5 minutes. Stir in the couscous and cook, stirring often, until toasted and golden brown, 1 to 2 minutes. Add 2 cups hot water and bring to a boil. Reduce the heat to low, cover and cook until the couscous is tender, about 12 minutes.
Halve the peppers vertically and remove the seeds and membranes. Gently rub the remaining teaspoon olive oil over the peppers and place on a baking sheet. Roast until the peppers are tender, 20 minutes.
Combine the cooked lentils and couscous in a large bowl along with the basil, feta, parsley, Kalamata olives, lemon juice, and salt and pepper to taste.
Divide the couscous mixture into each cooked pepper cavity, and garnish with extra basil and feta, if using.