In a large skillet over medium-high heat, add the butter. When the butter has melted, add the pear slices, onions, lemon juice, and a pinch each of salt and pepper. Cook until the pears are warmed through and are soft but not falling apart, about 5 minutes.
Place the toast squares on a baking sheet. Lay a few slices of pears in a single layer on each toast square. Top each with the crumbled blue cheese. Place under the broiler until the cheese begins to melt, 1 to 2 minutes. Transfer to a platter, garnish each with a sprinkle of the white chocolate shavings, and serve immediately.