Run a lime wedge around the rim of a margarita or rocks glass and dip in salt. Fill the glass with fresh ice.
Add jalapeño slices and a couple small watermelon chunks to a shaker; gently muddle to release heat and juice.
Add tequila, fresh lime juice, triple sec, and simple (or agave) syrup. Add ice to the shaker.
Shake hard for 10–15 seconds until the shaker is icy cold.
Fine‑strain into the prepared glass over fresh ice. Garnish with a lime wedge and a watermelon wedge.
Notes
Use silver tequila to let the watermelon shine. Start with one jalapeño slice and add more to taste. For a frozen version, blend everything with ice instead of shaking. A splash of fresh orange juice softens the heat nicely.