Preheat oven to 425 degrees. In a large saucepan, melt 2 Tbsp. butter. Add flour and garlic; cook, whisking for 1 minute (do not let brown). Gradually whisk in milk; bring to boil. Reduce to a simmer; cook until sauce has thickened, 2-3 minutes. Stir in green beans and artichoke hearts; season with salt, pepper and a pinch of cayenne. Transfer to a shallow 2 quart baking dish. Melt remaining butter. Toss butter with breadcrumbs. Scatter over green bean mixture. Bake until crumbs are golden brown and sauce is bubbling, about 10 minutes.