Heat oil in a 12-inch heavy skillet over moderately high heat until hot but not smoking, then sauté onion and shallots, stirring, until lightly browned, about 2 minutes.
Reduce heat to moderate and continue to cook, stirring occasionally, until softened, about 8 minutes more.
Add chopped spinach and cook, stirring, just until wilted, about 2 minutes.
Remove from heat and cool slightly.
Transfer spinach mixture to a bowl, then stir in cream cheese, sour cream, salt, and pepper until combined well.
Chill, covered, at least 1 hour.

Cooks' note:
Dip can be chilled up to 1 day (though the spinach will lose some of its color).
Let stand at room temperature 15 minutes before serving.