Preheat the oven to 400ºF/205ºC. Arrange two oven racks, one on the bottom and one in the middle.
Cook the lentils. Bring a Dutch oven or large saucepan of water to a boil. Add a pinch of salt, then lentils. Reduce heat to maintain a simmer for about 8 minutes. Taste a lentil—it should be cooked through but with a bite to it (you don’t want it to get soft). Drain well and transfer to a large kitchen towel to dry out. You can gently run your hands through the lentils to help them dry quicker (it takes about 10 minutes). Prep and roast the squash. While the lentils are cooking, slice the squash in half, vertically, then scoop out the seeds in the cavities (no need to peel the squash).* Take a paring knife and make some crosshatch marks in the flesh, but don’t cut all the way through to the skin. Drizzle each half, flesh side up, with some of the olive oil and rub in. Season the flesh with salt and pepper. Line a rimmed sheet with parchment paper. Place flesh side down and roast on bottom rack for 50 to 55 minutes, or until somewhat browned and tender.
Roast the lentils. Add the lentils to a second rimmed sheet pan and toss with 1 TBSP olive oil and sprinkle with a few pinches of salt and pepper. Spread the lentils out and shake the pan back and forth to evenly distribute so there’s no overlap. Set aside for now.
When the squash has 25 minutes left, add the pan of lentils to the middle oven rack. Roast for about 25 minutes, or until browned and crisp, shaking the pan halfway through to toss.
Cook the aromatics. When the squash has about 15 minutes left**, heat 2 TBSP of the olive oil in the saucepan used earlier over medium-high heat. Once hot, add the onions and season with a pinch of salt. Cook for about 7 minutes, or until softened and turning color. Add the garlic and cook for another 2 minutes, stirring frequently. Add the Spice Mix and stir for 30 seconds. Pour in the broth, scraping up any browned bits. Bring to a simmer and simmer for 5 minutes.
Toast the bread (now or before serving the soup). If you have a few minutes before the squash is done, toast your bread using a toaster oven. Alternatively, you can toast your bread in a frying pan with a bit of olive oil and toast on both sides until golden brown. Or, you can wait until the squash is done roasting and toast the bread in the oven for a few minutes on both sides.
Blend the soup. When the squash is done, remove skin (it should come off easily with your hands). Transfer flesh to a stand blender. Carefully pour in the onion-broth mixture and season with 1 heaping teaspoon kosher salt, lots of black pepper, and red wine vinegar. Remove the center cap and cover with a dish towel to allow steam to vent. Turn the blender to low, then slowly increase speed and blend for a few minutes until the soup is smooth, creamy, and warmed through.*** Taste the soup seasonings and add more salt/pepper or vinegar or lemon juice as needed.
Pour the soup into bowls, top each with crispy lentils, and chopped cilantro and green onions, if using. Drizzle a bit of olive oil on each slice of bread, sprinkle with a pinch of salt, and serve alongside the soup.