Spray an 8 quart slow cooker with a non stick spray.
Add onions, potatoes, carrots and celery to the bottom of slow cooker.
Place roast on top of veggies.
Sprinkle with salt, pepper, garlic salt, thyme and onion soup mix over meat.
Pour diced tomatoes on top of meat and add broth.
Cover and cook on low 10-12 hours or on high for 7-8 hours.
One hour before roast is done cooking, add frozen corn and green beans, cover and continue cooking for additional hour or until heated thoroughly.
Mix corn starch with water and stir into sauce to thicken it up a bit.
Serve and enjoy!