Melt 1 Tablespoon of Challenge Tuscan Style Spreadable Butter in a skillet over medium heat. Sauté steak strips then transfer them to a dish using a slotted spoon and cover to keep warm. Melt an additional 3 tablespoons of Challenge Tuscan Style Spreadable Butter in the skillet, add mushrooms and onions; cook until soft (about 10 minutes). Stir in mustard.
Meanwhile prepare rigatoni according to package directions (about 10 minutes in rapidly boiling water). Drain and combine with remaining 4 tablespoons of Challenge Tuscan Style Spreadable Butter.
Combine flour, salt and black pepper in a small cup, and slowly stir in cream to create a slurry. Add the flour slurry and sherry to skillet; cook until thicken.
Add sautéed steak strips and sundried tomatoes; heat for an additional minute. Divide pasta on individual serving plates, top with sirloin strips and sauce. Serve warm with a garnish of basil sprigs.