Combine the cherries, sugar, mustard seeds, white wine vinegar, and salt in a small skillet. Simmer over medium heat for 8 minutes, stirring now and then, until the cherries soften and the liquid thickens slightly. Set the mostarda aside to cool.
Warm the shredded duck confit in the same skillet over low heat for 3 to 4 minutes until heated through. Stir gently to keep the meat in nice shreds.
Spread butter on both sides of each brioche slice. Toast the slices in a skillet or under the broiler until golden on both sides. Cut each slice into four triangles.
Top each brioche triangle with a small spoonful of warm duck confit. Add a dab of cherry mostarda on top and finish with a few fresh thyme leaves.