Collect about 350 pieces dandelion heads, making sure to remove the green stems and keep only the full yellow blossoms.
Place the flowers in a bowl and cover them with a few cups of water. Let them soak for a while so that any insects hidden inside can float to the surface and be removed.
Once cleaned, transfer the dandelion heads to a pot and pour over 4 cups water, cold. Add slices of 1 lemon to the mix.
Bring everything to a gentle boil, then reduce the heat and let it simmer for 20 minutes. After simmering, remove the pot from the heat and let it sit at room temperature—preferably overnight—to allow the flavors to deepen.
Strain the cooled mixture through a clean piece of cotton cloth, squeezing out as much liquid as possible. Discard the leftover petals and lemon slices.
Pour the strained liquid back into the pot and bring it to a boil once more. Lower the heat and gradually stir in 3 cups granulated sugar, mixing until the sugar has fully dissolved.
Let the mixture simmer gently on very low heat for another 2 hours, skimming off any foam that forms on the surface. This slow simmering helps the syrup thicken to a honey-like consistency.
Allow the finished dandelion honey to cool slightly, then pour it into clean, sterilized jam jars. Seal each jar with a lid and store in a cool, dark place.