In a small frying pan over medium-high heat, cook pancetta, stirring often, until crisp, 2 to 3 minutes.
Transfer to a paper towel to drain.
Discard all but 1/2 tbsp. fat from pan.
Add shallot and stir over medium-high heat until soft, 1 to 2 minutes.
Remove from heat and whisk in oil, vinegar, mustard, and thyme. Season with salt and pepper.
Put spinach and sliced pear in a large bowl and gently toss with dressing. Arrange salad on plates. Crumble goat cheese on top and sprinkle with pancetta.