Set the oven at 350°F Rinse the chicken inside and out with cold water. Drain all of the water out and pat the chicken dry inside and out with dish towel or paper towel. Rub it inside and out with olive oil. Now sprinkle inside and out with salt and pepper then rub it in. Deeply puncture the lemons with the tip of a paring knife or any other sharp pointy tool and put the lemons inside the bird. Squash the garlic cloves just enough to break the skin. Remove the skin and put the garlic inside the bird.
Use toothpicks or trussing needle and string seal shut the front and back openings of the chicken.Put the chicken in a roasting pan breast facing down. Don't add any fat, oil or any other liquid.The chicken will baste itself. Put it in the middle of the pre heated oven. After a 30 minutes turn it over to have the breast side facing up.
Cook it for 30 minutes more. then turn the oven up to 400° and roast it for 20 minutes more getting the skin crispy. To check to see if it is done I suggest a meat thermometer poked into the thickest part of a thigh being sure that it doesn't touch the bone. When it reads 175° it is done. If you don't have a meat thermometer just gently pull the leg sideways from the body and look down into the space between the thigh and the body. When there is no red or pink to be seen, only brown it is done.
You could cut it up into serving size pieces and spread them onto a platter. Or, you could put the whole chicken on the table to be carved in front of everyone. Which ever you do, have the juices from the baking pan ready to top off the servings.
Yes, you can do it in a slow cooker. 4 hours on high or 8 hours on low.