Moms Beef Shish Kabobs

Name: Mom's Beef Shish Kabobs
Ingredients: 0.5 cup soy sauce; 1/3 cup vegetable oil; 0.25 cup lemon juice; 1 clove garlic, minced; 1 tablespoon prepared mustard; 1 tablespoon Worcestershire sauce; 1.5 teaspoons salt; 1 teaspoon coarsely cracked black pepper; 1.5 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