In a medium saucepan, combine the rice and stock or water. Bring to a boil, cover, reduce heat to low and cook until the water is absorbed, about 45 minutes. Fluff the rice and let it cool down.
Preheat the oven to 375°F. Cut the zucchini in half lengthwise. Use a spoon and scoop out the seeds, leaving about 1/2-inch (1 cm) of the zucchini on all sides.
Heat 2 tbsp (30 mL) of olive oil in a large skillet over medium-high heat. Add the onion and cook until translucent – about 5 minutes. Stir in the garlic and chili flakes and cook for another minute or two.
Remove from the heat and stir in the herbs, lentils, feta cheese, olives and cooked rice. Season to taste with salt and pepper.
Divide the rice mixture evenly between the zucchini shells. Spread your favourite tomato sauce on the bottom of a 9×13 baking dish. Arrange the stuffed zucchini over the sauce. Brush the edges of the zucchini with olive oil and drizzle some over top of the zucchini. Bake until the zucchini is tender when pierced with a fork. This could take anywhere from 20-45 minutes, depending on the size of your zucchini. If you find the rice mixture getting too dark, cover the zucchini with aluminum foil.