Preheat oven to 500 degrees Fahrenheit.

Cut tomato and onion in half.

Coat tomato and onion halves with vegetable oil.

Place oiled tomato and onion in a small roasting pan.

Roast for approximately 30 minutes, shaking the pan and turning vegetables occasionally.

Transfer roasted tomato and onion to a 5-quart casserole dish with a tight-fitting lid.

Add carrots, celery leaves, mushrooms, parsley stems, garlic, cabbage (if using), bay leaf, dried thyme, dried oregano, and water to the casserole dish.

Cook, covered, in a high-power microwave oven at 100 percent power for 30 minutes.

Remove casserole dish from microwave and uncover.

Strain the broth through a fine sieve to remove solids.

Add kosher salt to the strained broth and stir until dissolved.

Serve warm or store for later use.