In small bowl, combine yeast and water. Stir gently to combine. Set aside until frothy, approximately 5 minutes.
In a large bowl, combine high-gluten or bread flour, millet flour, millet, salt, baking powder, and sugar. Stir to combine.
Slowly incorporate yeast-water mixture into flour mixture, stirring until a dough forms. Refrigerate overnight.
Remove dough and set aside in a warm place for 1 hour. Place a baking stone in the center of the oven. Preheat oven to 450 degrees.
Divide dough into 6 equal balls. Roll each ball into an 8- x 9-inch oval. Prick with a fork 10 times. Brush each oval with 1 tablespoon olive oil and sprinkle lightly with salt and pepper. Top each with ¼ cup cheese and ¼ cup olives.
Bake each flatbread on the preheated baking stone until crust is golden and cheese is melted and bubbly, 8 to 12 minutes. Top each flatbread with ½ cup spring greens. Cut flatbread into bite-size pieces. Serve warm.
