Directions

1. Prepare your Napoleon P500 RSIB gas grill for high-heat, direct grilling.
2. Marinate portobello mushrooms in balsamic vinegar for 15 minutes. During this fifteen minutes, prep your vegetables and condiments. Slice your red onion into 1/4"-thick slices, keeping the rings intact (do not separate). Slice your tomatoes into 1/4"-thick slices as well.
3. To make the basil mayo, take prepared mayonnaise and blend it with freshly washed and dried basil.
4. Finely grate parmesan cheese and preheat a cast-iron skillet using the side burner over medium heat. Once hot, put down a layer of grated parmesan about the diameter of your burger bun. Place the top half of the burger bun on top of the melting cheese. Toast the cheese/bun in the pan until crispy and the cheese is more golden in color. Do this to all the top halves of each bun and set them aside until ready to assemble.
5. Remove your portobellos from the balsamic marinade and set them on a wire rack. (You can reuse the marinade to make a balsamic vinaigrette, or to dress a Caprese salad.) Season with both sides of each portobello with Cattleman’s Grill Italiano Seasoning. (Cattleman's Grill Tuscan Seasoning would also work well here.) Allow 5 minutes for Cattleman's Grill Italiano Seasoning to set onto the portobello.
6. Grill seasoned portobellos over direct heat until heated through (200°F is a good number for serving them hot!).
7. To build the burger, take the bottom bun and spread on a layer of basil mayo. Then, follow up with an heirloom tomato slice, thick red onion slice, the finished portobello, a little arugula, and top with the parmesan bun.
8. **Recipe Notes: This can easily be turned into a vegan recipe if the mayo is substituted with a vegan-friendly mayo (such as Veganaise) and the parmesan cheese is omitted or replaced with a dairy-free version (such as Follow Your Heart brand).