Arrange an oven rack 5 in. from heat and preheat broiler. Cook pasta according to package instructions; drain well, then rinse with cold water. Drain again and set aside.
In a small bowl, mix panko, parmesan, oil, and thyme. Set aside.
Melt butter in a large ovenproof frying pan over medium-high heat. Add flour, salt, pepper, and cayenne; cook, whisking, until bubbling and light tan, 1 1/2 to 2 minutes. Add milk and cook, whisking, until small bubbles appear at edge of pan and sauce just begins to thicken, 3 to 5 minutes. Reduce heat to low, add cheddar, and whisk until blended. Add crab and reserved pasta; toss gently to mix and heat through.
Spread crab mixture evenly in pan and scatter with panko mixture. Transfer to oven and broil until bread crumbs are well browned, about 1 minute. Top with green onions.
Wine pairing: Cinder 2015 Dry Viognier (Washington and Idaho; $18).