Stir onions into large non-stick frying pan sprayed generously with cooking spray until tender but not brown. Add chopped mushrooms, salt to taste, and saute for 5 minutes. Pour in 1/2 cup vegetable broth and balsamic vinegar simmering uncovered until reduced and absorbed. There should be no liquid in the pan and vegetables should be soft, but not brown.
While the mushrooms cook, combine mango and brown sugar with 2 tablespoons of water to prevent burning in a small pot for about 10 minutes until it thickens. Pulse with immersion blender leaving some small chunks but making it spreadable. Put into shallow bowl to cool.
To assemble panini: Take jelly roll pan that the matzoh will fit into and fill with 1/2 cup water. Immerse matzo sheet gently without cracking and hold down in water just until softened. Remove from water and lay on even surface such as a towel- lined counter. The matzo should feel soft, but still remain firm to handle. Spread 2 matzos sheets with a total of about 1 tablespoon mayonnaise. Over the mayonnaise, spread one side with mushroom mixture and the other with mango jam. Divide the jam and mushrooms evenly among all sandwiches spreading ingredients across matzo sheets. Close sandwich. Give the same large fry pan a good dose of cooking spray and place sandwich on hot pan. Fry on medium heat using a flat pot cover or a smaller pan bottom to gently press down. You want the matzo a bit crispy so listen for the sizzle sounds. Raise or lower heat as needed. Spray top of sandwich with cooking spray and using a spatula, gently flip sandwich. Press again and fry on medium high heat for just about 2 minutes. Repeat process for the each sandwich.