Preheat your oven to 400 degrees Fahrenheit. Place the halved butternut squash on a lined baking sheet, cut side down. Roast for 45-50 minutes, or until soft. It may take more or less time depending upon how large your squash is, but you want to be able to scoop out the flesh without losing the shape of the squash when you stuff it.
While your squash is roasting, prepare the filling. Place the lentils in a saucepan with the bay leaf and enough water to cover by 2 inches. Bring to a boil and simmer for 20-25 minutes or until just starting to soften. Drain, remove the bay leaf and set aside once done.
Heat a large skillet over medium heat. Add the fennel and celery and sauté for 10 minutes, or until starting to soften and lightly brown. Next add in the chopped mushrooms and continue cooking for another 5 minutes. Finally, mix in the fennel fronds, rosemary and seeds, along with the leafy greens and cook for a few minutes to just wilt the greens. Turn off the heat, then stir in the cooked lentils and set aside.
Once the squash is cooked, let cool slightly and scoop out the flesh, leaving about a half an inch inside the skin to support the structure. Mix the scooped out squash into the lentil and mushroom stuffing, then add to fill each half of the squash. You may have extra, so just save that for leftovers.
Bring the temperature down to 350 degrees Fahrenheit and return the stuffed squash back into the oven for about 20 minutes to warm through. Before serving, drizzle a bit of butternut squash seed oil over top for a nutty addition to the final product, or omit if you are oil-free.