Add spinach and red onions to a large bowl.
Place the pancetta in a medium saute pan, then place over medium heat and toss around until it starts to give up its fat. Lower the heat to medium-low and continue cooking until the pancetta is browned and crisp, about 10 minutes.
Use a slotted spoon to transfer the pancetta to a bowl and set aside.
Add the shallots to the pan and cook until they just start to soften, about 1 minute.
Add the mustard, vinegar, salt and pepper, then lower the heat and whisk until emulsified. Taste for seasoning and add more olive oil or vinegar if needed.
pour dressing over the spinach and toss until evenly coated. Add the crumbled goat cheese and reserved pancetta, then toss until evenly distributed.
Serve immediately with more freshly cracked black pepper on top.