Heat the broiler.
Cook pancetta in a medium skillet over medium-high heat until crisp, about 2 minutes. Add spinach and cook, stirring occasionally, until just wilted, 1 to 2 minutes.
Spread ½ teaspoon mustard on each slice of toast. Top each with the pancetta and spinach mixture. Scatter the shredded cheese on top.
Transfer the toasts to a baking sheet and broil until the cheese has melted, about 2 minutes. Serve warm.