Mom's Beef Shish Kabobs

INGREDIENTS

• 1/3 cup vegetable oil
• 1/2 cup soy sauce
• 1/4 cup lemon juice
• 1 tablespoon prepared mustard
• 1 tablespoon Worcestershire sauce
• 1 clove garlic, minced
• 1 teaspoon coarsely cracked black pepper
• 1 1/2 teaspoons salt
• 1 1/2 pounds lean beef, cut into 1-inch cubes
• 16 mushroom caps
• 8 metal skewers, or as needed
• 2 green bell peppers, cut into chunks
• 1 red bell pepper, cut into chunks
• 1 large onion, cut into large squares