Preheat the oven to 400°F. Place asparagus on a parchment-lined baking sheet. Drizzle with olive oil, then sprinkle with salt and pepper. Roast in the oven for 15-20 minutes, until tender but not mushy.
While the asparagus is roasting, toast the pine nuts in a small skillet over medium heat until they start to brown, making sure to toss frequently to avoid burning. Remove from heat and place in a small bowl.
When the asparagus is done, remove from oven and transfer onto a serving plate. Zest the lemon over top, then cut the lemon in half and squeeze the juice over the asparagus (you may use the other half of the lemon, juiced, if desired). Garnish with watermelon radishes, toasted pine nuts, and fresh mint leaves (if desired) and serve.