Add your butter to a medium sized sauce pan and melt over medium heat. Whisk constantly until golden brown. Immediately remove from heat and transfer to a heat safe glass container and reserve until ready for use. Trim the ends off the brussels sprouts, and cut in half lengthwise, in quarters if necessary to balance the size for even cooking. Add 3 tbsp olive oil to a cast iron pan and heat over medium-high heat. Arrange the brussels sprouts cut side down in the oil. Turn the heat to medium, and sear on one side until Golden brown browned, about three minutes. Turn the brussels sprouts over and cook on the other side until nicely browned and tender, three to five minutes. Add your lemon juice and 2 tbsp of your prepared brown butter and toss until evenly coated. Remove from the heat, season with salt and pepper. Skewer Brussels, brush each with a light coating of brown butter, top with feta, and garnish with fresh thyme and lemon zest. Serve immediately.  Enjoy!