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1
Trim chicken breasts to a uniform size, shape and thickness; they should be about 1/2 inch thick.
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2
(If using larger breasts, slice them into 1/2-inch-thick cutlets.)
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3
Season generously on both sides with salt and pepper.
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4
Velvet the chicken: In a medium-size mixing bowl, whisk egg whites until frothy, then whisk in cornstarch until lump-free.
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5
Add chicken breasts and coat well with cornstarch mixture, then cover and marinate for 30 minutes.
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6
(Chicken may also be marinated several hours ahead and refrigerated.)
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7
Meanwhile in a small mixing bowl, stir together Dijon mustard, whole-grain mustard, dried mustard, horseradish and creme fraiche.
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8
Set aside.
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9
Put butter or oil in a skillet over medium heat.
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10
Lay chicken breasts in the pan and saute very lightly for 30 seconds a side without browning.
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11
Remove breasts and blot on paper towels.
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12
Discard remaining oil and wipe pan.
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13
Return skillet to stove and add mustard and creme fraiche mixture and chicken broth.
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14
Whisk together to make a thin sauce and bring to a gentle simmer.
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15
Add breasts and simmer for 1 minute, then turn breasts over, cover pan and simmer for 2 minutes.
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16
Turn off heat and leave covered for 1 or 2 minutes more, until meat is firm to the touch.
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17
Transfer breasts to a warm platter.
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18
Bring sauce to a boil, add chopped thyme and reduce until slightly thickened.
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19
Spoon sauce over breasts, sprinkle with chives and serve.
