Remove the sausage from casings. Discard the casings. In a large saute pan, heat the olive oil and then brown the Italian sausage while crumbling it with the back of a wooden spoon. When cooked through, add the red onion and saute for 3 minutes. Add the garlic, fennel seed, salt, pepper, oregano, and basil to the sausage and onion mixture. Mix well and saute 1 minute. Next, add the mushrooms and bell pepper and saute for 3 minutes. Remove from heat and stir in the olives.
Preheat panini press. Place one slice of bread on a plate or cutting board to assemble the panini. Add 2-3 slices of cheese (depending on bread size) in a single layer. Add approximately a half a cup of the topping mixture followed by 2-3 more slices of cheese in a single layer.
Spray the preheated panini press with spray oil. Place the sandwich inside the press, lower the lid, and gently press. Cook until bread has reached your desired crispness and cheese is melted inside. About 3-5 minutes. Cut panini across into two triangles. Serve with a bowl of marinara for dipping.
*Bread should be something sturdy, not inexpensive sandwich bread. Rustic loaves sliced into sandwich thickness slices work well. Experiment with different flavors and textures such as: whole grain, roasted garlic, and sourdough.