1 teaspoon finely grated lemon zest
1 teaspoon finely grated Meyer lemon zest
1 tablespoon fresh lemon juice
1 tablespoon fresh Meyer lemon juice
1 tablespoon whole grain mustard
1/4 cup olive oil, plus more
Kosher salt, freshly ground pepper
2 large bunches thick asparagus, trimmed
2 bunches spring onions, halved if large
Prepare grill for medium-high. Whisk lemon zest, Meyer lemon zest, lemon juice, Meyer lemon juice, mustard, and 1/4 cup oil in a medium bowl to combine; season dressing with salt and pepper (if you don’t have Meyer lemons, use regular lemons in their place).
Place asparagus and spring onions on a rimmed baking sheet and lightly drizzle with oil. Season with salt and pepper; toss to coat. Grill, turning once, until lightly charred and crisp-tender, about 4 minutes. Serve drizzled with dressing.