Heat oil in a large pot over medium heat. Add onion and cook, stirring occasionally, until golden, 8 to 12 minutes.
Meanwhile, separate chard greens from stems. Thinly slice the stems; chop the greens.
Add garlic, cumin and Aleppo to the pot; cook until fragrant, about 30 seconds.
Add water, lentils and the chard stems; bring to a boil over high heat.
Reduce heat and simmer, stirring occasionally, until the lentils are falling apart, about 20 minutes.
Stir in the chard leaves and cook until tender, about 5 minutes.
Stir in salt, lemon juice and pepper before serving. Garnish with more oil and Aleppo, if desired.
