Melt butter in a large high-sided skillet over medium; add onion, and cook, stirring often, until softened but translucent, about 6 minutes. Add garlic, and cook, stirring constantly, until fragrant, about 1 minute. Add lamb, and cook, stirring often and breaking up meat using a wooden spoon, until lightly browned, 6 to 8 minutes. Add wine; reduce heat to medium-low, and simmer until reduced by half, 6 to 8 minutes. Add stock, and simmer until reduced by half, 6 to 8 minutes. Add tomatoes with juices, breaking up loosely using a wooden spoon; stir in fresh oregano, coriander, bay leaves, and 2 teaspoons of the Espelette pepper.