In a medium pan over medium heat, add butter to melt. Add garlic, onion, and green peppers. Cook for about 2-3 min, or until soft. Add the rosemary and tomatoes, sauté until fragrant. Add beer and water, bring to a simmer, add mustard and mix thoroughly, then add clams.
Cover and steam for 3 min.
Season with salt and pepper to taste and garnish with parsley.
