Heat the oil in a saucepan over medium heat until warm. Add the shallots and cook gently, stirring often, until soft and nearly translucent, about 3 minutes. Whisk in the vinegar, Dijon, and maple syrup. Continue whisking over medium heat until the dressing thickens slightly. Remove from the heat and taste for seasoning.
Place spinach and apple slices in a bowl and pour warm vinaigrette over. Toss lightly and then top with a very generous grating of Gruyère. Serve immediately.
Yields 4 servings