1. In a large, heavy-bottom sautée pot heated over medium-high heat, add the oil and saffron. Bloom saffron, then stir in the onions, leeks, garlic, pepper. Cook, stirring frequently, until the onion is translucent and the herbs are fragrant, 6 to 8 minutes.
2. Stir in the wine and bullion cubes, scraping up brown spots. Add tomatoes, tomato sauce. Simmer gently, uncovered, to develop the flavors, 30 to 45 minutes. The cioppino base can be made ahead of time to this point; remove from heat, cover and refrigerate up to one day (reheat before continuing).
3. To the pot, add the shrimp, scallops and fish. Cover and simmer gently just until the fish and shellfish are firm and opaque, about 10 minutes. Uncover the pot and remove from heat.
If adding mussels or clams, sautée steam in wine/water mix in a separate skillet until open. Discard cooking water.