First, gather all the ingredients. Prep ingredients as needed, for example: wash lettuce and wipe clean mushrooms and cut mushrooms into long strips about 1/8th of an inch.
Next, make the mushroom marinade/dressing.
Then use 6 tbsp of the mushroom marinade to marinate the mushrooms. (2 tbsp of mushroom marinade is reserved to toss/dress the sprouts and lettuce.) I recommend either a plastic bag or a dish where the mushrooms get equal access to the marinade. Mushrooms act as a sponge and will quickly soak up the marinade, so the mushroom slices must get access to equal marinade.
While the mushrooms are marinating, make the two other sauces and slice the avocado. (The sweet chili mayonnaise is a must, but the sriracha cream cheese is optional if you're short on time and ingredients.)
Then, grill the mushrooms. Grill the mushrooms on medium-high heat for 1-2 minutes per side. (You can grill the mushrooms in multiple ways. Easily in a saute pan on the stovetop. In a grill pan on the stovetop. Or on any outdoor BBQ or grill.)
While the mushrooms grill, toss the sprouts and the lettuce in the reserved 2 tbsp of marinade. (This reserved marinade has now become a dressing to add flavor to the greens.)
Once the mushrooms are done, remove them from the grill or grill pan onto a plate or plater.
Set up an assembly line for filling the ingredients to stuff the pitas.
How I recommend assembling a pita: cut open a pita it a serrated knife. Add a tablespoon of sriracha cream cheese to the bottom half of the pita. Then add a 1/2 tbsp to 1 tbsp of sweet chili mayo to the top half of the pita. (If you omitted the sriracha cream cheese add sweet chili mayo to both sides.) Next, add the grilled portobello mushroom to the bottom half of the pita right on top of the sriracha cream cheese. Then, dressed sprouts, then avocado slices, and finally the dressed lettuce.