Combine 0.5 cup sugar and 0.5 cup water in a small saucepan. Warm over medium heat, stirring until sugar dissolves completely. Remove from heat and let cool about 5 minutes.

Blanch parsley (optional, brightens color)

Bring a small pot of water to a boil. Add the packed parsley leaves for 10–15 seconds, then immediately transfer to an ice bath to stop cooking. Drain and squeeze out excess water. This step mellows raw grassy flavor and intensifies green color; skip if you prefer raw herb brightness.

Blend sorbet base

In a blender, combine the cooled simple syrup, blanched (or raw) parsley, 2 tablespoons fresh lemon juice, 1 teaspoon light corn syrup (if using), and a pinch of salt. Blend on high until completely smooth and bright green, about 30–45 seconds. Taste and adjust lemon or sugar for balance (should be slightly tart and herbaceous).

Chill and freeze

Strain the parsley purée through a fine-mesh sieve into a bowl, pressing with a spatula to extract as much liquid as possible (this yields a smoother sorbet). Cover and chill the liquid thoroughly in the refrigerator (30 minutes). Freeze in an ice cream maker according to manufacturer instructions until a soft-sorbet consistency is reached (about 20–30 minutes). Alternatively, pour into a shallow metal pan and freeze, stirring vigorously with a fork every 30 minutes until scoopable (2–3 hours). Transfer to a container and harden in freezer for at least 1 hour before serving.

Cocktail Assembly (5–7 minutes)

Chill a coupe or flute glass in the freezer for a few minutes or rinse with ice water and discard.

Place one generous scoop of parsley sorbet into the chilled glass.

In a cocktail shaker filled halfway with ice, combine 1 tablespoon Saint Germain and 0.25 oz fresh lemon juice. Shake vigorously for 8–10 seconds to chill and slightly dilute.

Strain the shaken Saint Germain and lemon into the glass over the sorbet.

Slowly top with 4 oz well-chilled Prosecco, pouring gently to avoid excessive foaming and to create a light effervescent layer.

Garnish with a small parsley sprig and optionally a lemon twist. Serve immediately with a small spoon and/or stirrer so the guest can fold the sorbet into the bubbles as they drink.