- Preheat oven to 400F. Slice each croissant lengthwise into 4 thin slices. Toast until golden brown. Set aside to cool.
- Meanwhile, in large non-stick or cast-iron skillet, heat oil over medium-high heat. Add mushrooms; sprinkle with salt. Cook, stirring occasionally until well-browned, about 12-15 minutes.
- In bowl, combine Dijon, vinegar, horseradish, garlic, sage and pepper. Scrape mixture into pan and mix until combined. Add butter.
- Continue to cook, stirring, until mushrooms are no longer wet, about 1 minute more. Remove from heat.
- Top each toast with mushrooms and sprinkle with chives.
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