Prepare the Grapefruit Juice

Roll the grapefruit on the countertop applying gentle pressure to loosen the juices and maximize yield. Halve the fruit and juice it using a citrus juicer to extract fresh, pulpy nectar. Strain through a fine-mesh strainer to remove seeds and excess pulp. If you prefer a sweeter cocktail, dissolve 1 tablespoon of sugar in the juice and stir until completely dissolved. Set aside.

Chill Your Glassware

Place 4 champagne flutes in the refrigerator for at least 30 minutes before serving. For a quicker method, fill the flutes with ice water and let them sit for 3-5 minutes, then pour out the water and dry the glasses just before assembling the cocktails.

Assemble the Cocktail

If using ice, add 1-2 ice cubes to each chilled champagne flute. Measure and pour 2 ounces (1/4 cup) of fresh grapefruit juice into each glass.

Add the Champagne

Tilt each champagne flute at a 45-degree angle. Slowly pour the Champagne down the side of the glass to preserve the bubbles and prevent overflow. Fill each glass to about 3/4 full, leaving room for garnish. Pour gently to maintain the effervescence.

Garnish and Serve

Place a fresh grapefruit slice or wedge on the rim of each glass. If using rosemary, gently slap the sprig between your hands to release the aromatic oils, then place it in the glass for an herbal fragrance. Serve immediately while the bubbles are lively and the drink is ice cold.