Put a small plate in the freezer for testing the set later.
Add the cherries, vinegar, water, sugar, serrano and bell pepper to a deep, heavy-bottomed saucepan. Stir over medium heat until the sugar dissolves, then bring to a boil.
Boil steadily, stirring often, for about 20 minutes, until the cherries are soft.
Take the pan off the heat and let it sit for 5 minutes. Then use a hand blender to blend the jam to your desired consistency (about half blended works best).
Return the pan to medium-high heat and boil, stirring often and scraping the bottom as it thickens, until it reaches 220°F on a thermometer, about 10 to 20 minutes more.
No thermometer? Spoon a little jam onto the cold plate and return it to the freezer for 1 minute. Push it with your finger: if it wrinkles, it's ready. If it's still runny, boil for a few more minutes and test again.
Ladle the jam into 2 clean half-pint (8-ounce) jars, leaving a little space at the top. Let it cool, then seal and refrigerate. It firms up more as it chills.