Coarsely chop the onions.
Heat a pot over medium heat and add the butter.
Sauté the chopped onions in the butter for a couple of minutes. They should not brown, just become translucent.
Add dried rosemary and let it sauté for a moment.
Add half of the water and half of the red wine to the pot with the onions. Add tomato paste and beef bouillon - make sure the bouillon completely dissolves in the liquid.
Boil the sauce thoroughly for 3-4 minutes.
Pour the sauce through a sieve into another pot. Add the rest of the red wine and water as well as gastrique.
Increase the heat until the sauce boils vigorously.
Let the sauce boil down over high heat until about 1/3 of it is left.
Season with cream, salt, and pepper. Be careful with the salt, as the bouillon is very salty. You may not need more salt; taste the sauce before adding any!
Shake flour and water together to make a slurry in a shaker. Pour the slurry into the sauce and let it boil for a couple of minutes to remove the flour taste.