Slice the baguette into ¼" thick slices. Brush each slice with olive oil and season with salt and pepper. Toast by grilling on high heat or baking in a preheated oven at 450°F (230°C) for about 7 minutes until golden and crispy.

Heat butter or olive oil in a hot skillet over medium-high heat. Add sliced mushrooms, minced shallot, and thyme leaves. Sauté for about 2 minutes until mushrooms soften and shallots are fragrant.

Pour in sherry vinegar to deglaze the pan, letting it simmer for about 30 seconds. Stir in flour and cook for another 30 seconds while stirring constantly.

Gradually add vegetable stock (or broth or water) to the skillet, stirring to combine. Simmer briefly until thickened. Adjust seasoning with salt and pepper. Remove thyme leaves if desired.

Spoon a couple of tablespoons of the mushroom mixture onto each toasted baguette slice. Garnish with fresh thyme leaves and a drizzle of olive oil.