Preheat oven to 400 F and line 2 rimmed baking sheets with parchment paper.
Lay mini Portobello mushrooms stem (40 pieces) side up in a single layer on the first baking sheet. Brush with 1/2 the olive oil, then sprinkle with salt and pepper. Bake in the preheated oven for 20 minutes.
Meanwhile, combine ground Italian sausage (1 pound), egg (1), and 1/2 cup grated parmesan (1/2 cup) together in a large bowl.
Roll the sausage into 20 balls, then press them into patties.
Place in a single layer on the second baking sheet.
After 20 minutes, grab the mushroom pan and flip them so they're now facing stem side down. Brush this side with the rest of the oil, and return to the oven.
At the same time, place sausage pan in the oven. Both will take 16-18 minutes to cook, you'll know the sausages are done once their internal temp reaches 165 F, and the mushrooms are done once they're fork-tender.
Remove the pans from the oven.
Place 1 quarter of cheese on top of each patty.
Return the sausage baking sheet back into the oven and bake until the cheese melts (about 5 minutes). Allow patties to cool slightly before assembling.
Place 2 basil leaves on the stem side of 1/2 the mushrooms. Place patty over the basil, then spoon spaghetti sauce on top. Finish with the other half of the mushrooms.
Place a toothpick in the center, or eat as is. Enjoy!