Slice the jalapeño and place 2 to 3 slices into a clean jar with a lid or a cocktail shaker. Pour tequila over the jalapeño and set aside to infuse for 10 to 20 minutes.

Meanwhile, moisten the rim of the glass with a lime wedge. Place fleu de sel or the spice mix on a flat plate and carefully dip the moistened rim of the glass into it, rotating it so the salt adheres. Tap off any excess salt if necessary and set the glass aside.

After 10 to 20 minutes, add some ice cubes to the screw-top jar and fill it with orange juice, lime juice, and agave syrup. Tightly seal the jar or cocktail shaker and shake vigorously for approximately 20 to 30 minutes.

Place 3 to 4 ice cubes into the prepared glass and pour the margarita through a strainer into the glass to catch the jalapeño and chili seeds. Garnish the spicy margarita with a fresh jalapeño slice and serve immediately.