In a high-sided saucepan (or soup pot) over medium heat, heat the olive oil then cook the onion and garlic, stirring often, until the onion is translucent (2 to 3 minutes).
Add the carrots and jalapeño pepper to the saucepan and continue cooking, stirring occasionally, until the carrots have softened (approximately 7 to 8 minutes).
Add the coriander, turmeric, cumin, allspice, salt, and pepper. Cook, stirring often, for 1 or 2 more minutes.
Add the vegetable broth, plum tomatoes, and red lentils - stir to combine. Lower the heat to low and allow the soup to simmer for 30 minutes, stirring occasionally, until the lentils are softened.
Using a hand food processor or blender, blend the red lentil soup to your desired consistency. It can remain chunky or be totally smooth.
Add more salt, black pepper, and crushed red pepper (optional, if you want it spicier) to taste, then serve.
Notes
Want it spicier? Try a fresh serrano pepper instead of a jalapeño. Serranos are typically double the heat.