Begin by prepping the rim of your cocktail glasses with Tajin (optional).
In a cocktail shaker, muddle together your serrano pepper with fresh lime juice. Use a wood muddler to gently crush the peppers into the lime juice. Next, add tequila, orange liqueur and agave nectar. Then add ice.
Shake for 10-15 seconds until icy cold.
Strain and pour into your prepared glass filled with fresh ice. Garnish with a lime slice and several serrano pepper slices.
Notes
More Spice - For a higher spice factor, add more seranno peppers to muddle in the cocktail shaker.
Less Spice - For less spice, remove the seeds from the peppers before using.
Tool Substitutions - If you don't have a wood muddler, you can use the back of a wooden spoon or any wooden utensil.
Rim Idea - If you don't have Tajin for your rim, try a spicy salt rim. Combine coarse salt with a little chili pepper and use that for your rim.
Secret Ingredient - It is well known (so not so "secret" I guess?) that the magic ingredient that makes a homemade margarita come alive is fresh lime juice. The other magic ingredient is adding a splash of orange juice. You can add that to this recipe it you want to amplify the flavor a little more.
Juicer - I rarely pull out my juicer at home, a hand juicer is all you need for this recipe (and all cocktail recipes that involve citrus juice).