Preheat oven to 550° F or as high as your oven will go.
Over medium low heat, caramelize the onions in the butter until soft and golden brown.
Using tongs, remove the onion to a paper towel-lined, leaving as much butter behind in the pan as possible.
Add the mushrooms to the hot pan and brown.
Then add the thyme to the mushrooms and stir to heat.
Turn off the flame and begin assembling the flatbread pizzas on a section of parchment paper.
Brush olive oil over the tops of the flatbread pieces, being careful to reach to the edges.
Spread half of the diced garlic over each, distributing evenly.
Sprinkle 1/4 cup of shredded mozzarella over each.
Then layer the onion and mushrooms.
Drop half slices of the roast beef in small piles.
Sprinkle some more mozzarella lightly over the roast beef.
Lift the parchment paper with the assembled flatbread pizzas onto a cookie sheet.
Place on the middle rack of the pre-heated oven.
Bake until the cheese is melted and begins to turn golden brown, and the bread edges are becoming crispy, about 2-3 minutes
Remove from the oven and place on a cutting board.
Rest for about 3-5 minutes, then slice and serve.