Slice the fresh jalapeño into rings. Drop 1-3 rings to the bottom of a cocktail shaker or a glass. Use a muddler or the handle of a utensil to smash the jalapeño.
Add ice, tequila, lime juice, and agave nectar to the shaker or glass. Mix or shake to combine.
Use a lime wedge to wet the rim of a glass (or two if splitting this recipe...see notes). Spread the flake salt on a small plate and then dip the glasses in the salt to coat. Fill the glasses with ice.
Pour the margarita from the cocktail shaker or mixing glass evenly between the two glasses (if splitting) or one glass if serving as a Grande Margarita. Garnish with lime wedges and an additional jalapeño slice or 2 if desired.
NOTE: This recipe makes one Grande Margarita (about a full pint glass) or two smaller margaritas (as pictured).