Tip:The wonton cups will keep, unfilled, in an airtight container for up to a week. Fill them right before serving or they'll get soggy.
Position a rack in the center of the oven and heat the oven to 375°F. Using mini (2-inch) muffin tins, press the wonton wrappers down into the tin, laying back the corners to make a defined cup. Bake until lightly browned, 8 to 10 minutes. Transfer to a rack and let cool.
Meanwhile, season the beef liberally with salt and pepper. Turn on the exhaust fan. Heat an ovenproof sauté pan over high heat until very hot. Add the beef and sear, rolling it onto all sides in the dry pan until it's lightly browned, 3 to 4 minutes total. Transfer the pan to the oven and cook until medium rare (130°F on an instant-read thermometer), 5 to 10 minutes depending on the thickness of the cut. Let the beef rest for 5 minutes and then cut it into julienne (long, thin strips).
Combine the beef, red pepper, and scallions in a medium bowl. In a small bowl, combine the lime juice, fish sauce, sugar, garlic, and chile paste and stir until the sugar dissolves. Toss with the beef mixture. Fill the cooled wonton cups with the filling, about 1 Tbs. per cup.
A few of the wonton cups may collapse during baking; be prepared to bake a few extras, just in case.