Add lemons, mint leaves, stevia, and water (not the sparkling water) into a high-speed blender and blend on high. If you don't have a high-speed blender, you might want to pour the lemon-mint concentrate over a strainer or a nut milk bag.
At this point, you can either add the sparkling water to the concentrate right away and adjust the taste or put the concentrate in the fridge and add the sparkling water right before serving (so the water stays fizzy).
Serve chilled over ice with a few mint leaves and slices of lemon.
*This lemonade tastes great even without the mint leaves. So add as many (or as few) mint leaves as you like. I usually add just a few to taste. **Start with 4 cups of sparkling water and add more to taste.