In a large Dutch oven, heat oil at medium. Add onions, carrots, and garlic and cook 7-10 minutes, or until just beginning to brown, stirring frequently. Add flour and cook for 2 more minutes. Add tempeh and cook 2 minutes more.
Add beer and raise heat to high. Boil for 3 minutes. Add broth, sweet potato chunks, lentils, and herbs.
Reduce heat to medium-low and simmer for 40 minutes, or until lentils and sweet potatoes are tender and sauce begins to thicken. Season with salt and pepper to taste.