- 8 ounces button mushrooms
- 1 medium yellow onion, peeled
- 1 large green or red bell pepper
- 1 1/2 pounds beef sirloin, fat trimmed by the butcher or an adult
- 1 tablespoon Baby Bam 
- 1/4 cup Worcestershire sauce
- 1/4 cup soy sauce
- 2 tablespoons balsamic vinegar
- 2 tablespoons vegetable oil
- 1 tablespoon chopped garlic
Place the mushrooms in a large bowl of water. Gently turn them in the water to remove any grit or dirt. Quickly remove and dry the mushrooms on a clean dish towel. Set aside.
Cut the onion in half. Cut each half into quarters, keeping the pieces together as much as possible.
Cut off the upper quarter of the bell pepper. Remove and discard the stem end, inside ribs, and seeds. Cut the pepper into quarters lengthwise, then into 1-inch pieces.
Cut the meat into 1-inch pieces and place in a large glass bowl. Add the Baby Bam and toss to coat. Add the Worcestershire sauce, soy sauce, vinegar, oil, and garlic, and stir to combine. Cover the bowl tightly with plastic wrap and refrigerate for at least 2 hours and up to 4 hours.
Make sure the oven rack is in the top position and preheat the oven to 450°F. (Or, alternatively, preheat a grill to medium-high.)
Line the baking sheet with aluminum foil.
Remove the meat from the refrigerator. Carefully thread one meat cube onto a skewer, followed by a mushroom, a piece of bell pepper, and a chunk of onion. Continue threading alternating ingredients onto the skewer until it is full. Place the filled skewers on the baking sheet.
Transfer the baking sheet to the oven and bake the kabobs for 10 minutes.
If grilling, place the skewers on the grill and cook, turning as necessary, until meat is cooked to the desired degree of doneness, usually 10 to 12 minutes for medium to medium rare.
Using oven mitts or pot holders, remove the baking sheet from the oven. With a mitt or tongs, hold one end of a skewer. With the other hand use a fork to push the meat and vegetables from the skewer onto a plate. Repeat with the remaining skewers and serve hot.
You really have to use either a glass bowl or a resealable plastic bag to marinate the meat. Metal will react with the marinade, giving the food a funny flavor.