Make the dough: In a medium bowl, whisk yeast and sugar into warm water to dissolve. Let stand until foamy, about 5 minutes. Add flour and salt and stir with a wooden spoon until a shaggy dough is formed. Knead on a lightly floured work surface until dough is slightly tacky. Be careful not to incorporate too much flour while kneading.
Lightly oil a large bowl. Transfer dough to bowl and cover with plastic wrap. Let stand in a warm place until nearly doubled in size, 1 to 1 1/2 hours.
Place a pizza stone on floor of a gas oven (remove racks) or bottom rack of an electric oven. Preheat oven to 475 degrees for at least 45 minutes.
Make the topping: Place bacon in a medium skillet and cook over medium-high heat, stirring, until lightly browned and crispy, about 7 minutes. Using a slotted spoon, transfer bacon to a paper towel-lined plate and set aside.
Drain all but 1 tablespoon bacon fat and place skillet over medium-low heat. Add onions and season with salt and pepper; cook, stirring occasionally, until soft and translucent, about 12 minutes. Remove onions from heat and set aside to cool.
In a small bowl, mix together creme fraiche, egg, and coriander; season with salt and pepper. Set aside.
Punch down dough to deflate; divide into 4 equal pieces. Working with one piece of dough at a time (keeping remaining dough pieces lightly covered with plastic), roll out dough on a lightly floured work surface into a rectangle about 10 inches by 8 inches, stretching with fingers if necessary. Using a pizza cutter trim a scant 1/8 inch from edges to make even.
Transfer dough to a lightly floured pizza peel or the back of a baking sheet. Using an offset spatula, spread one-quarter of the creme fraiche mixture evenly over dough, spreading as close to the edge as possible. Top with one-quarter of the bacon, onions, cheese, and thyme.
Gently shake pizza on peel to ensure that pizza will slide easily onto pizza stone. Open oven door and align the front of the peel with the back of the stone; swiftly jerk peel backward to release pizza onto one side of the stone. Bake, rotating with a large spatula halfway through, until crust is crispy and golden, 8 to 10 minutes.
Remove pizza from oven and top with additional creme fraiche and thyme leaves. Repeat process with remaining pieces of dough. Cut into squares and serve immediately.