Preheat oven: Set oven to 400°F (200°C). Line a baking sheet with parchment paper.
Roast the pears: Place pear halves on the prepared baking sheet. Drizzle lightly with olive oil and sprinkle with salt, pepper, and thyme if using. Roast for 20–25 minutes, flipping halfway, until the pears are tender and slightly caramelized. Let cool slightly before slicing.
Toast the walnuts: In a dry skillet over medium heat, toast the walnuts for 3–4 minutes, stirring occasionally, until fragrant and lightly golden. Remove from heat and let cool.
Prepare the dressing: In a small jar or bowl, whisk together olive oil, balsamic vinegar, Dijon mustard, honey, salt, and pepper until emulsified. Taste and adjust sweetness or acidity as needed.
Assemble the salad: In a large serving bowl, add arugula. Layer roasted pear slices on top, followed by goat cheese crumbles, toasted walnuts, and red onion slices.
Dress and serve: Drizzle with balsamic dressing or glaze just before serving. Toss gently or leave arranged for a more elegant presentation.