Wash and pat the cherries dry. Remove the stems and pit them (see Note 1).
Add the sugar, lemon juice and 2 tablespoons of water.
Turn on the heat to low and cook the cherries until the cherries turn soft. Keep stirring the fruits constantly to avoid the bottom from burning. As you stir, try breaking them with the spoon. This should take about 10 minutes, depending on the heat you use. If the mixture turns too dry and the cherries are still not soft, you can more water and continue to cook them until the cherries are soft.
Let the cooked cherries cool down and then add them into a blender. Add two and half tablespoons of water and process them for about 10 to 15 seconds (see Note 2).
Pour the blended cherries puree into a strainer.
Use the back of a spoon to press out as much extract and sauce as possible from the cooked fruits, leaving on the skin pulp behind. Discard the pulp. (see Note 3).
Return the strained cherry coulis into the pot.
In a small plate, mix the cornstarch with the remaining half tablespoon of water. Make sure it is smooth and there are no lumps of the cornstarch.
Add the cornstarch mix into the coulis.
Turn on the heat and stir the cornstarch quickly to ensure the cornstarch does not cook into gooey lumps in the cherry coulis.
Continue cooking over low heat until the coulis thickens to your desired consistency.
Turn off the heat and the the cherry coulis cool down to room temperature before pouring it into a clean container. Keep the coulis refrigerated until ready to use.