Heat 1 tablespoon neutral oil in a large pot over medium-high heat. Add the onion, celery, and carrots and cook, stirring regularly, for 7–8 minutes until just beginning to sweat and soften. Season lightly with salt and pepper.
Add the roasted red bell peppers and matbucha if using, and toss to coat.
Melt the butter into the vegetables. Once bubbly and melted, add the tomato paste and crushed red pepper. Stir to coat the vegetables in the paste and cook for 2 minutes.
Pour in the water or stock and bring to a boil. Reduce heat to medium-low and hard simmer for 25–30 minutes. Stir occasionally and add more liquid as needed or reduce the heat if the stew reduces too quickly—taste and season with salt and pepper. The stew should be thick, not soupy, so be careful not to add too much water.