How to Make It
Preheat oven to 375ºF. Line 2 large baking sheets with parchment.
In a large saucepan over medium-high heat, bring 1 cup water, butter, mustard, salt and pepper to a rolling boil. When butter has melted, add flour all at once. Stir vigorously with a wooden spoon until a sticky ball forms and it begins to pull away from sides of pan, about 3 minutes. Lower heat and cook, stirring, for 3 minutes. Remove from heat, transfer to a mixing bowl and let cool for 5 minutes.
Using an electric mixer on medium speed, beat in 1 egg. Let mixture separate and then come back together before adding next egg. Repeat with remaining eggs. Beat in cheeses and thyme.
Portion dough by tablespoonfuls onto baking sheets, spacing 1 inch apart. (Alternatively, use a piping bag with a plain tip or a small ice cream scoop.) Bake until golden brown, 25 to 30 minutes, switching pans front to back and top to bottom halfway through. Serve warm.