Heat the oil in a large skillet or saucepan over medium-high heat. Add the onion and cook until soft and translucent, about 3 to 5 minutes. Add the garlic and cook for another 30 seconds.
Add the rice and stir to fully coat with the oil. Cook for three minutes.
Add the wine and cook until almost all the liquid has evaporated, about 3 to 5 more minutes.
Transfer the rice mixture to the slow cooker. Add the heated stock or broth, lemon zest, lemon juice, salt, and pepper. Cover, then turn on HIGH and cook for 90 minutes.
Remove the lid and stir the rice with the liquid. There will still be plenty of liquid at this point, but the rice should have expanded and starting to become soft. Stir in the asparagus, cover, and continue cooking on HIGH for another 30 minutes.
Remove the lid and stir in the butter and Parmesan cheese. Taste and add more salt, if desired.
Serve at once, adding a drizzle of quality olive oil, a sliver or two of fresh Parmesan, and a couple of grinds of black pepper, if desired.