Pour the tequila into a mixing glass or small pitcher. Add the sliced jalapeño, ¼ to ½ of it, depending on how spicy you want your margaritas. Set aside at room temperature for 15 minutes (the longer you let the jalapeños infuse, the spicier it will be).Make the margaritas by mixing together the jalapeño-infused tequila, Grand Marnier, lime juice and ¼ cup simple syrup or orange juice. Taste. If you’d like a spicier margarita, add more jalapeño and let it it sit a little longer. Pour into glasses and add lime slices and jalapeños for garnish. For a lighter margarita, fill the glasses ½ full and top with sparkling water.