Heat remaining 1 Tbsp. oil and butter in a large skillet over medium heat. Once melted, add Brussels and salt; toss to combine. Arrange Brussels, cut side-down, and cook, undisturbed, for 4 minutes, until golden-brown. Add chiles and toss with Brussels. Continue to cook, stirring occasionally, until Brussels are crisp-tender, about 4 to 5 more minutes. Pour in maple-mustard mixture and cook 1 minute, stirring constantly, until mixture throughly coats sprouts. Stir in thyme.