Preheat oven to 350 F (180C). This is to bake the risoni later!
In a large saucepan, heat the olive oil over medium heat. Add the onion and mushroom, and saute several minutes until soft.
Add the salt and pepper, stir, then add the garlic and cook for another minute. Add the risoni and stir, toasting for a minute.
Sprinkle the flour on top of this mixture, and stir, then add the vegetable broth and milk.
Bring to a boil then reduce the heat and simmer uncovered for 9-11 minutes (or according to package instructions) until the risoni has cooked al dente. Keep stirring frequently to avoid the bottom burning, and if pasta absorbs too much liquid while cooking, just add a splash of water
Remove from heat, stir in the spinach and the Parmesan cheese.
Transfer the whole mixture to a baking dish, and top with panko breadcrumbs and mozzarella cheese. Bake for 20 minutes until cheese is golden and bubbling, then if you want a deeper color, broil for another 1-2 minutes.
Let the dish rest for just a couple minutes, then enjoy hot!