Preheat your oven to 180C (350F).
Wash and dry peppers well. Cut peppers in half from stem to tip and carefully remove seeds and white ribs from the insides. Leave the stem intact. Lay peppers on a lined baking sheet until required.
Rinse lentils and quinoa well. Place in a small saucepan with the vegetable stock. Bring to the boil over medium-high heat. Once boiling, reduce to a simmer and fit the saucepan with a lid. Allow to simmer for 15 minutes. Then remove from the heat. Stand for five minutes before draining and fluffing up with a fork. Keep at room temperature until required.
Finely dice onion, celery and carrot. Heat the olive oil in a small saucepan. Saute onion, celery and carrot until the onion is translucent and the carrot is softened. Add thyme, rosemary, and oregano and remove from the heat. Stir this mixture into the lentils and quinoa.
Add sliced olives to the stuffing.
Spoon stuffing into prepared peppers. Crumble feta over the stuffed peppers.
Place peppers into the oven and bake for 45 minutes or until the peppers are soft and wrinkled.
Serve with a green salad.