This method can be used for beef, chicken, fish and vegetables.
Heat oven to about 400 degrees F. Put cleaned beef, chicken or fish into a large baking pot. Add any fresh vegetables you care to, including carrot tops, if desired. Carrots, celery, and bell peppers are good to use. Onions do not need to be peeled. Place in the oven and brown for about 45 minutes.
Remove from the oven. Place roasted meat and vegetables in a stockpot. Add spices, as desired, such as garlic cloves, whole peppercorns, parsley, bay leaf, thyme and/or rosemary. Don't add salt, as you normally add this to the dish you are preparing which uses the stock. Cover the meat and vegetables with double their volume in water. Bring the pot to a boil, then reduce the heat to low. Leave the pot uncovered so that some liquid evaporates and intensifies the taste.
Seafood stocks need only 1 hour to develop maximum flavor, but beef and chicken should simmer at least several hours. Cook the stock until about one-third of the original liquid remains. Strain it, let it cool, and degrease it.
The stock can be frozen in ice cube trays and cubes used as needed.
For vegetable stock, simply omit the meat, chicken or fish from the above instructions.