Preheat the oven to 425 degrees F.
Combine the squash, 2 tablespoons of olive oil and 1/2 teaspoon salt on a parchment-lined baking sheet. Toss to combine and arrange in an even layer. Bake in the oven until browned and caramelized, about 30 to 40 minutes. Set aside.
Meanwhile, heat 2 tablespoons olive oil in a large cast iron skillet or paella pan over a medium-high flame. Add the onion and saute until soft, about 5 minutes. Toss in the leeks and continue to cook until wilted, about 3 more minutes. Add the garlic, paprika, chili flakes, and rice. Stir to coat in the oil and onion mixture. Continue to cook, stirring occasionally, until all the rice has gotten a chance to toast on the bottom of the pan, about 3 minutes. Add the wine and scrape up any brown bit on the bottom of the pan. Stir in the tomatoes and 1 teaspoon salt. Spread the rice mixture in an even layer in the pan.
Pour enough of the stock over the rice so it is just slightly submerged. Bring to a boil, then turn the heat down to medium-low and simmer until most of the liquid is absorbed, about 25 to 30 minutes. Taste the rice; it should be crusty around the edges and al dente on top, but cooked through in the middle. If not, add another cup of stock and continue to cook for another 5 to 10 minutes.
When paella is done, arrange the roasted butternut squash on top and garnish with parsley or scallions.