In a small saucepan, stir together cherries, ½ cup water, and lemon juice. Bring to a boil, reduce heat, and simmer for 20 minutes. Remove from heat.
In a medium saucepan, melt butter over medium heat. Add onion; cook until caramelized, approximately 45 minutes, stirring occasionally. Remove from heat. Stir in 2 tablespoons red wine, scraping browned bits from bottom of pan.
Add caramelized onion to cherry mixture, stirring to combine. Transfer mixture to a heatproof bowl, and let cool to room temperature.
In a medium bowl, stir together short ribs, garlic, and remaining 2 cups red wine. Cover and refrigerate for at least 30 minutes.
Remove short ribs from marinade, and pat dry with paper towels. In a medium sauté pan, heat oil over medium-high heat. Add short rib mixture; cook until meat has browned completely, 5 to 8 minutes. Remove from heat.
Divide cherry-onion mixture and short ribs among crostini.
Garnish with watercress, if desired. Serve immediately.
Notes
Make-Ahead Tip: Cherry-onion mixture can be made 3 days in advance, stored in an airtight container, and refrigerated. Let come to room temper-ature before using.
Recipe by TeaTime Magazine at https://teatimemagazine.com/cherry-short-rib-crostini-recipe/