2 teaspoons extra-virgin olive oil
6 ounces spinach or chard, washed
Salt and pepper
3 ounces thick-cut bacon, cut crosswise into 1/4-inch lardons
4 large 1/2-inch-thick slices bread from a day-old rustic loaf, such as miche or pain au levain (about 6 by 4 inches)
4 ounces Raclette or Gruyère cheese
Put oil in a skillet over medium-high heat. Add spinach, season with salt and pepper, and cook, stirring for about 1 minute, until just wilted. Drain, cool and squeeze out excess water.
Fry bacon lardons in a dry skillet over medium-high heat for about 1 minute, until barely crisped. Remove and drain on paper towel.
Put the bread slices on a baking sheet and toast lightly on both sides under broiler (or use a toaster and place toasted slices on baking sheet). Brush lightly with mustard. Divide greens among the four toasts and spread out to cover. Tuck the bacon here and there.
Top each toast with slices of Raclette or Gruyère (about 1 ounce total on each toast). Broil for 2 to 3 minutes, not too close to the heat, until nicely browned. Alternatively, bake tartines 3 to 5 minutes on top shelf of a 450-degree oven, or individually in a countertop toaster oven. Serve hot.