Preheat the oven to 375°F. Arrange the baguette slices on a baking sheet and toast for 8 to 10 minutes until lightly golden and crisp.
While the bread toasts, place the shallot, cherries, sugar, and red wine vinegar in a small saucepan. Bring to a simmer over medium heat and cook for 6 to 8 minutes until the cherries soften and the liquid thickens slightly.
Remove the saucepan from the heat and stir in the Dijon mustard until fully combined. Set the mostarda aside to cool for a few minutes.
Slice the smoked duck breast thinly against the grain into 12 pieces.
Top each toasted baguette slice with one piece of duck, then add a small spoonful of cherry mostarda. Finish with a few fresh thyme leaves.