Place the chopped rhubarb, granulated sugar, apple cider vinegar, water, sweet chili sauce, grated fresh ginger, red chili flakes, and salt into the slow cooker.
Stir all ingredients together until well combined.
Cover and cook on low for 4 hours, stirring occasionally to prevent sticking.
After 4 hours, use an immersion blender to carefully blend the mixture until it reaches a jam-like consistency, or leave it slightly chunky if preferred.
In a small bowl, mix the cornstarch with cold water to create a slurry.
Stir the cornstarch slurry into the slow cooker jam mixture.
Turn the slow cooker to high and cook uncovered for an additional 15-20 minutes, stirring frequently, until the jam thickens.
Remove the slow cooker insert from heat and stir in the lemon juice.
Let the jam cool slightly before transferring it to sterilized jars.
Store the jam in the refrigerator and use within 3 weeks.