Put a sheet pan or pizza stone in the oven to get it heated up too.
Combine the Sancerre, honey, and lemon juice in a saucepan and place over medium-low heat. Simmer gently, swirling the pot around, until the wine is syrupy and coats the back of a spoon, about 15 to 20 minutes.
Butter both sides of each slice of bread. Put a nice hunk of bleu cheese on top of the baguette slices and arrange them on the hot sheet pan or stone. Bake for 5 minutes until the cheese melts and the bread toasted.
Put the lettuce in a mixing bowl and toss with the olive oil, salt, and pepper. Pile the greens on a serving platter with the bleu cheese crostini around it. Drizzle the wine syrup over the crostini and greens and sprinkle with the walnuts.