Easy Vegan and Gluten-Free Stuffed Mushrooms

40 white button mushrooms, stems removed
1-1/4 cups plus 2 tablespoons chickpea flour
3/4 cup water
2 tablespoons minced chives
1/8 teaspoon celery salt
1 teaspoon salt
2 garlic cloves, minced
1 teaspoon baking powder
3 sage leaves, minced
1/2 cup finely chopped mustard greens (leaves), dried well
Olive oil, for garnish