Mash the garlic cloves until they form a fragrant, garlicky paste. Then add the sardine fillets, salt, and pepper, and mash them together. Set this mixture aside.
In a heavy-bottomed pot or Dutch oven, add the olive oil and sauté your chopped onions, diced mushrooms, and celery over medium heat. Let them cook until the onions turn translucent, about 5-7 minutes.
Add in the rosemary, thyme, oregano, and chili flakes, letting them bloom in the heat for around a minute.
Add the can of diced tomatoes and their juices to the pot, and let them simmer for approximately 5 minutes, infusing the stew with rich tomato flavors.
Stir in the sardine and garlic paste, making sure to incorporate it evenly. Let this fragrant blend cook for an additional 2 minutes, allowing the flavors to meld.
Pour in the water and add the potatoes, then crank up the heat to bring it all to a boil. Once that boil is achieved, reduce the heat, cover and let the stew simmer until the potatoes are tender and easily pierced with a fork, this takes approximately 15 to 20 minutes.
Once the potatoes are done, gently place the cod or your choice of white fish into the stew and let it cook for 5 to 10 minutes, depending on the thickness of the fish pieces. They should flake easily with a fork when they're done. Make sure your stew isn't boiling when you add in your fish.
Serve this hearty dish in cozy bowls, accompanied by slices of toasted baguette. Enjoy!