Trim the woody ends off the asparagus, snapping where they naturally break or cutting about 2.5 cm / 1 inch from the bottom. Pat the spears dry.

Toss the spears with olive oil, salt, and pepper in a large bowl until evenly coated.

Heat a 30 cm / 12-inch cast-iron grill pan (or an outdoor grill) to medium-high, about 200 C / 400 F. Lay the spears in a single layer, spaced apart so steam can escape.

Grill for 4 to 6 minutes, turning once with tongs, until char lines form and a spear bends slightly without flopping, still crisp at the center. Transfer to a platter and tent loosely with foil to keep warm.

Fill the saucepan with about 5 cm / 2 inches of water and bring to a bare simmer. Set a heatproof bowl over the pan without letting it touch the water.

Whisk the egg yolks, water, and lemon juice in the bowl until pale and slightly foamy, about 1 minute.

Whisking constantly, drizzle in the warm melted butter in a slow stream. Keep whisking until the sauce thickens enough to coat the back of a spoon and holds a ribbon when drizzled, about 4 to 5 minutes; the yolks should reach roughly 160°F/71°C, warm enough to cook through but not curdle. If the eggs look grainy or start scrambling at the edges, pull the bowl off the heat immediately and whisk in a teaspoon of cold water to cool it down.

Season with a pinch of cayenne and salt to taste. The sauce should look pale yellow and glossy, not curdled.

Spoon the warm hollandaise over the grilled asparagus right away.

Finish with a crack of black pepper and a little lemon zest. Serve right away while the sauce is still warm and the asparagus is still crisp.