Combine honey, water, and red pepper flakes in a small saucepan over low heat. Warm gently, stirring, until honey thins and infuses, 2–3 minutes — do not boil. Remove from heat and let steep 10 minutes for more heat. Strain out pepper flakes into a small jar. Yield: about 1 tbsp infused hot honey.

Prepare glass

Rub a grapefruit wedge around the rim of a highball or Collins glass. Mix Tajin and optional coarse salt on a small plate and dip the rim to coat. Fill the glass with ice.

Build the cocktail

In a shaker filled with ice combine 2 oz tequila, 2 oz fresh grapefruit juice, 0.5 oz fresh lime juice, 0.25 oz fresh lemon juice, and 1 tbsp hot honey. Shake vigorously 10–15 seconds until well chilled and honey is incorporated.

Strain and top

Strain the shaken mixture into the prepared glass over fresh ice. Top with 2 oz club soda (or sparkling water) and gently stir once to combine.

Garnish and serve

Thread or float a grapefruit wheel or wedge on the rim or in the glass. Taste and, if desired, add another 1/4–1/2 tsp hot honey (stir to dissolve) for extra heat-sweetness. Serve immediately.

Notes: To make a larger batch, scale all ingredients proportionally; hot honey keeps refrigerated for up to 2 weeks. For a spicier infusion, increase red pepper flakes or steep longer. Use freshly squeezed juices for best brightness.