Spray the slow cooker liner with nonstick cooking spray. In a large skillet heat the olive oil over medium-high heat. Add the onion and cook until softened, about 3 to 5 minutes.
Stir in the garlic. Add the rice into the skillet and cook for 3 minutes, stirring often to avoid burning. Add the wine or 3/4 cup of your broth into the skillet and stir, cooking until almost completely absorbed.
Pour the rice into the slow cooker. Heat the rest of your broth in the microwave, pour that in. If you want to add lemon juice and lemon zest add that + salt and black pepper.
Cover the slow cooker and cook on high for 90 minutes. Remove the lid and gently fold in the asparagus if you want to add that. Continue cooking for an additional 30 minutes.
Prior to serving, fold in the butter and Parmesan cheese.