Rinse and dry the mushroom caps and trim the stem (no need to hollow them out).
Heat a large shallow pan and when hot, add the mushroom caps and sear for 2-3 minutes on each side (no need for added oil if the pan is really hot!!). Turn off the heat and set aside onto a plate until ready to assemble.
They will give out some liquid as they cool off in the plate:Keep the reserved liquid for later!
Making the stuffing:
Heat olive oil in large shallow pan over medium heat. Add garlic, onion, carrot and celery and sautee for 2-3 minutes.
Add the dry bulgur wheat to the pan and continue to cook while stirring for a few seconds and add the fresh (or dry) sage, salt and pepper.
Add the water and cook until tender and all the liquid is absorbed (about 5-8 minutes).
Remove from heat and stir in ground walnuts and reserved liquid from the cooked mushrooms.
Assembling:
Place the seared mushrooms on a baking sheet lined with parchment paper and fill (actually the stuffing just sits on top) each with the stuffing mixture, using an ice-cream scooper, packing down slightly. It will be overstuffed, that’s ok!