Brush one side of each slice of bread with olive oil. Place the mushrooms in a row on the unoiled side of the bread, slightly overlapping (the mushrooms will shrink slightly after grilling).
Brush the mushrooms with olive oil and season with salt. Place the bread, mushroom side up, in a preheated panini-style grill or between a hot cast iron skillet and press.
Grill with your equipment of choice and apply enough pressure to slightly smash the bread.
Grill for about 3 minutes, until the bread is golden brown and the mushrooms are sizzling and cooked through.