Heat the olive oil in a medium saucepan, add the onion, and cook until the onion softens, about 4 minutes. Add the rice and toast the grains, stirring occasionally, until the rice begins to look opaque (bright white), about 4 minutes. Add the water, butter, lemon slice, and lemon zest, reduce the heat to low, cover, and simmer for 25 minutes.
Meanwhile, slice off the tough ends of the asparagus stalks and discard them. Cut the spears on a diagonal into 1-inch lengths. Fill a large non-reactive pot with a steamer rack with 1/2 inch water, place the pot over medium heat, and bring the water to a simmer. Add the asparagus, cover, and cook until just tender, 4 to 5 minutes.
Transfer the cooked rice to a serving bowl and toss in the asparagus. Season with salt and pepper and serve.
Recipe courtesy of David Page and Barbara Shinn's RECIPES FROM HOME (Artisan, 2001)