Combine flour and oil in a large saucepan (off heat). Whisk until smooth. Place saucepan over medium-high heat and cook until it becomes golden-bronze hue, stirring constantly. (I use a wooden flat-edged spatula to cook the roux.)
When the roux is a golden-bronze color, add the onions and bell pepper. Sprinkle with ¼ teaspoon salt. Reduce heat to medium-low. Cook until peppers are soft, stirring often, about 10 minutes. Add minced garlic. Cook, stirring constantly, for 1 minute.
Add tomato paste, stirring constantly for one more minute. Stir in tomato sauce.
Slowly add clam juice or water, stirring well to incorporate into roux/tomato mix. The mixture will be thick, so stir often.
Add remaining ¾ teaspoon salt, black pepper, and cayenne pepper. Reduce heat to simmer. Simmer covered for 20 minutes, stirring occasionally.