Remove stems and cores from mushrooms; chop and reserve for later. In a frying pan, add olive oil and saute chopped onion. Add garlic as onion becomes pale. Mix together the onion and garlic mixture with the chopped stems and cores of mushrooms.

Tear up bread slices into bite-sized pieces and place in a small blender and pulse until crumbled. Place breadcrumbs in a bowl and toss with with enough olive oil to bring the mixture together. Stir in Italian Seasonings and transfer to a pan and toast briefly.

Mix together the onion and mushroom with the breadcrumbs. Stuff caps of mushrooms with mixture and top with Parmesan cheese. Place in the oven for 20 to 25 minutes at 350 degrees.

Spread pesto on each cracker, add onions and mushrooms, top with a slice of gruyere. Bake in 350 degree oven 5 to 10 minutes until cheese melts.