Combine in a sauce pan, the olive oil, onions, garlic, tomato paste, ketchup, chicken stock (or water with cube), parmesan cheese, sugar and oregano. Allow to simmer for 30 minutes.
Make the onion sauce:
Combine the onions, peppers and salt in a bowl or jar, fill with vinegar until level. This is good in the refrigerator for weeks. It’s the perfect topping for the Garnaches and is also good on sandwiches.
To make the Garnaches:
Toast your tostadas, spread refried beans on the tostada, add the sauce and onion sauce. Repeat, eat and enjoy!