Preheat oven to 200°C / 400°F.
Split your squash in half and scoop out the seeds and stringy bits.
Rub the squash with a bit of oil, and place the squash cut side down on a sheet pan lined with parchment paper. Roast for 15 - 20 min, until just barely tender.
While squash is roasting, prepare the stuffing.
Heat a small frying pan over medium-high heat. Add a tsp of olive oil, and sauté the onions and celery for about 5 minutes, until the onion is translucent. Remove from the heat.
Combine quinoa, lentils, onions and celery, cheese, tomato paste, and sunflower seeds to a large bowl, and give everything a good stir to combine.
When the squash is just becoming tender remove from the oven and cool slightly, just until you can handle it.
Spoon the filling in and press firmly in with your hands, making a mound of stuffing over the squash.
Return the squash to the sheet pan (or use a baking dish if your squash is tippy), stuffing side up, and return to the oven for an additional 15-20 minutes.
Squash is ready when it is fully tender and the stuffing is nice and crispy on top.
Serve warm or at room temperature.