Cut green beans in half if you like pieces to be a little smaller. Blanch the green beans: drop them into lightly salted boiling water and allow green beans to cook for about 3 to 4 minutes. Remove them from the boiling water with a slotted spoon and immediately plunge them into a bowl of ice cold water to stop the cooking process. Drain beans once they're cool and set aside.  Add bacon pieces to a skillet over medium heat. Cook bacon for two minutes, then add diced onion and garlic and continue cooking for 3 to 5 minutes, or until bacon is done (but not crisp) and onions are golden brown. Remove from heat and set aside. In a separate skillet or saucepan, melt butter over medium heat. Sprinkle flour into the pan and whisk immediately to evenly mix it into the butter. Cook for a minute or two, then pour in milk and half and half. Continue cooking, whisking constantly, while sauce thickens, about 2 minutes. Add salt, pepper, and cayenne then add the grated cheddar. Stir while cheese melts. If sauce is too thick, splash in some chicken broth as needed. Turn off heat.  Add pimentos to pan, then add bacon/onion mixture. Stir to combine. Pour over green beans and stir gently to combine. Pour into a baking dish and top with panko crumbs. Bake at 350 degrees for 30 minutes or until sauce is bubbly and panko crumbs are golden.