In a medium skillet heat oil over medium heat. Add mushrooms, sweet pepper, shallot, and garlic; cook mixture until pepper and shallot are tender, stirring frequently.
In a medium bowl combine sour cream, cream cheese for cooking, lemon peel, lemon juice, and mustard. Stir in mushroom mixture, artichoke hearts, and Gruyere cheese. Spoon artichoke mixture into a 1-1/2- or 2-quart slow cooker.
Cover and cook on low-heat setting for 2 to 2-1/2 hours or until heated through. Serve immediately or keep warm, covered, on warm-heat or low-heat setting for up to 2 hours. Stir before serving. Serve with toasted bread slices or pita chips.