Place the onion, potatoes, carrots and garlic in the bottom of a slow cooker. In a small bowl mix together the olive oil, garlic powder, onion powder, paprika, salt, and pepper to make a spice paste. Pat the outside of the chicken dry and slather the chicken all over with the spice paste.
Stuff the cavity with the rosemary, thyme and lemon, and place the chicken on top of the vegetables in the bottom of the crock pot.
Cover the slow cooker with the lid and cook on high for 4-5 hours or until the internal temperature inserted into the thickest part of the breast and thigh registers 165℉.
Remove the chicken and vegetables to a platter to rest. Squeeze out the roasted garlic cloves and add to the platter with the vegetables. Discard the papery husk of the garlic. Pour any juices that have accumulated in the bottom of the slow cooker through a fine mesh strainer into a liquid measuring cup, adding enough chicken broth to make 1 cup of liquid. Remove any extra fat from the liquid with a fat separator or use a spoon to spoon off the top layer of fat and pour this into a small saucepan on the stove, bringing it to a boil.
In a small cup, mix together the cornstarch with 2 tablespoons of cool water or broth until smooth. Pour this into the boiling broth and cook for 1-2 minutes stirring occasionally until gravy is thickened. Taste and adjust seasoning.
If desired for a crispy skin, you can gently scrape off extra spices (to keep them from burning) on the skin and brush with a little oil. Place the chicken under the broiler for just a few minutes to crisp and brown the skin. You could also place it into a 450℉ oven for 5-8 minutes. Slice chicken and serve with vegetables and gravy.