Use a garden spade to harvest 10 dandelion plants, leaving the roots in place.
Cut away the roots from the rest of the dandelion plants and rinse them clean with cool water.
Use a sharp knife to thinly slice the roots. The root pieces should be about the same size as green peas.
Spread the roots out in an even layer on a parchment-lined rimmed baking sheet and roast in a preheated 350 degree F oven for about 10 minutes.
Meanwhile, thoroughly rinse the 2 cups of dandelion flowers under cool water.
Add roasted dandelion roots and flowers to a small saucepan of boiling water, remove from heat, and let them steep for about 10 minutes until they come to room temperature.
Transfer tea through a fine-mesh strainer into a large glass pitcher and refrigerate for about 8 hours or overnight. The tea will grow a darker color the longer it sits in the fridge.
When ready to serve, make the simple syrup. Whisk 6 Tablespoons of sugar and 3 Tablespoons of warm water in a bowl until sugar has dissolved. Let cool completely, then stir into the dandelion tea until well combined.
Add lemon juice and lemon slices to bring out the sweet refreshing flavor of the dandelion tea and serve over ice cubes.