Using a vegetable peeler, peel lemon, leaving white pith behind; coarsely chop peel and set aside. Heat olive oil in a large skillet. Add onion and cook, stirring occasionally, until translucent, 8 to 10 minutes. Add garlic and cook until fragrant. Stir in beans, roasted red pepper, reserved lemon peel, and rosemary; add salt and pepper to taste. Add enough chicken broth to moisten the dish and bring to a simmer; cover and simmer over low heat until beans are quite tender, about 20 minutes. Taste and adjust seasoning; add additional lemon juice if desired.