Combine and bring to boil: Add 0.5 cups red lentils, rinsed, 2 cups low-sodium chicken or vegetable broth, 0.5 cups water, and 1 medium carrot, peeled and diced small to a small saucepan. Bring to a boil over medium-high heat.
Simmer: Reduce heat to low. Simmer uncovered, stirring occasionally, until lentils are completely broken down and soup is thick.
Season and finish: Stir in 0.3 teaspoons ground cumin (optional, only if tolerated) if using, 0.3 teaspoons salt, and 1 teaspoons fresh lemon juice. Taste and adjust salt. The soup should be fairly thick — add a splash of water if it tightens too much on standing.