Heat the oil in a medium skillet over medium heat until it shimmers. Add the garlic and onion and cook, stirring occasionally, until softened, about 5 minutes. Add the beans, broth, sugar, red pepper flakes and salt and black pepper to taste. Increase the heat to medium high and bring to a boil. Reduce the heat to maintain a brisk simmer and cook, stirring occasionally, until the liquid has reduced by about half, about 8 minutes.
Transfer to a food processor and pulse until almost smooth. Transfer to a bowl and stir in the chives, mint, parsley and lemon zest and juice; season with salt and pepper.
Trim the bottom 1/2 inch of the stem from the endives. Remove the large outer leaves and reserve for another use. Carefully separate the medium and small leaves, trimming the stem more as necessary. Reserve the baby leaves for another use.
Spoon some of the bean puree onto the base of the medium and small leaves and garnish each with a small mint leaf. Arrange decoratively on a platter. Serve immediately.