Preheat oven to 450 degrees, with racks in upper and lower thirds. On a rimmed baking sheet, toss rhubarb with honey. Roast on upper rack until beginning to soften, about 5 minutes. Let cool on baking sheet. On another rimmed baking sheet, toast walnuts on lower rack until fragrant, 5 minutes. Let cool, then chop.
In a large bowl, whisk together oil and vinegar and season with salt and pepper. Add arugula and fennel and toss to combine. Top with rhubarb, walnuts, and goat cheese.