Cut potatoes into 1-1/2-inch pieces. Place in microwave-safe dish; cover with vented plastic wrap. Microwave on HIGH 6 to 8 minutes or until just tender, stirring once. Cool slightly.
Cut beef sirloin into 1-1/4 inch pieces. Combine beef, mushrooms, tomatoes, potatoes, olive oil, and steak seasoning in a large bowl; toss. Alternately thread beef and vegetables onto metal skewers.
In a small sauce pot combine butter, parsley, garlic, and rosemary. Melt butter mixture over low heat either on the stove top or on your grill; until melted. stir occasionally.
Place kabobs on grid over medium, ash-covered coals. Grill, 9 to 12 minutes (over medium heat on preheated gas grill, 8 to 10 minutes) for medium rare (145°F) to medium (160°F) doneness, turning once and brushing with sauce during last 5 minutes. Remove from grill and brush with remaining sauce. Garnish with remaining rosemary, parsley mixture.
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