In a medium saucepan, combine the cherries and granulated sugar. Cook over medium heat, stirring occasionally, until the sugar dissolves and the cherries begin to soften, about 5-7 minutes.
In a small bowl, whisk together the cornstarch and water until smooth. Pour this mixture into the saucepan with the cherries.
Bring the mixture to a simmer, stirring constantly, until the sauce thickens, about 3-5 minutes.
Remove from heat and stir in the lemon juice and almond extract (if using).
Let the sauce cool slightly before serving. It will thicken further as it cools.
Notes
This sauce can be made ahead of time and stored in the refrigerator for up to a week. Reheat gently if desired.