Heat oil in a large skillet over medium-high. Add onion and 1/2 tsp. salt and cook, stirring occasionally, until caramelized, about 15 minutes. Stir in fennel and cook 30 seconds. Add beans, remaining 1 tsp. salt, and 2 Tbsp. water and heat through, stirring to combine. Transfer to a food processor or blender and purée until smooth.