Heat oil in a saucepan over medium heat. Add cherry tomatoes and garlic. Cook, covered, until tomatoes burst, about 5 minutes. Add chard; cook until wilted. Add water, lentils, and 1/2 teaspoon salt. Bring to a boil; cover and reduce heat to low. Simmer until lentils are tender yet firm to the bite, about 25 minutes. Stir in lemon zest and additional salt to taste.