Toast the walnuts in a dry frying pan until brown, about 2-3 minutes. Transfer to the bowl of a food processor. Add the olive oil, honey, and thyme and pulse until the walnuts are coarsely chopped. Season with salt and pepper to taste and pulse again until the walnuts are finely chopped.
Place the baguette halves cut side up on a baking sheet lined with parchment paper. Spread the walnut mixture over the tops, and top with the Havarti slices and grated Gruyere. Bake until the cheese is bubbly, about 5 minutes.
Let cool slightly then use a serrated knife to cut the baguette into slices about an inch thick. Top each crostini with a few arugula leaves, a piece of prosciutto, and a pear slice. Grind some pepper over the tops and serve warm or at room temperature.
