Bring broth and 2 cups water to a simmer in a medium saucepan (do not boil). Keep warm over low heat.
Heat 2 teaspoons oil in a large saucepan over medium-high heat.
Add minced onion, celery, and carrot; saut 1 minute.
Add rice; saut 5 minutes. Stir in wine; cook 5 minutes or until liquid is nearly absorbed, stirring constantly.
Add broth mixture, 1/2 cup at a time, stirring constantly until each portion of broth is absorbed before adding the next (about 20 minutes total).
Remove from heat; stir in Parmesan cheese and green onions.
Preheat oven to 37
Remove stems from mushroom caps; discard.
Place mushroom caps, gill sides up, in a 13 x 9-inch baking dish. Spoon 1 1/4 cups risotto mixture into each cap; top each with 1 tablespoon mozzarella cheese.
Pour 1/2 cup water into dish.
Bake at 375 for 30 minutes or until mushroom caps are tender.
Heat 1 teaspoon oil in a large Dutch oven over medium-high heat until hot.
Add chopped onion and garlic; saut 2 minutes or until tender.
Add spinach; saut 2 minutes or until spinach is wilted. Arrange 1/2 cup spinach mixture on each plate.
Remove stuffed mushrooms from baking dish with a slotted spoon; place on spinach mixture.