Heat butter and olive oil in a large pot over medium heat. Add onion and season with salt and pepper. Cook, stirring occasionally, until the onions are softened and starting to get a little color, 8–10 minutes.
Reduce heat to medium and add cumin and crushed red pepper flakes (if using). Stir to bloom the spices a bit in the fat, 90 seconds or so. Add the squash and garlic and season with salt and pepper. Cook, stirring every now and then, until the squash starts to fall apart (it should look like a very coarse mash), 15–20 minutes.
Add lentils, broth, bullion paste and season with salt and pepper. Bring to a gentle simmer, letting the squash melt into the broth as the lentils become tender and follow suit (by also melting into the broth), 30–35 minutes. The soup should be split-pea soup in texture, not entirely smooth (this isn’t a puree), but creamy with bits of squash here and there. If it feels watery or too thin for your soup preference, continue simmering until you’ve reached the texture that’s pleasing to you.
To serve, add the vinegar, ladle into each bowl and divide herbs among, stirring to let them wilt into the soup a little (which will really perk up the aroma, especially if using a mix of dill and cilantro like I would). Drizzle with a bit of olive oil, maybe a grind or two of pepper or chili flake. While it is creamy enough for me, if you want to spoon yogurt, sour cream, labne or creme fraiche over top, you can—I know some of you will anyway!