Julia Child's famous magnum opus, MTAOFC ("Mastering the Art of French Cooking") is reknowned for reducing many complex recipes to practice. Her (or should I say "their") recipe for fish soup is worth knowing.
At first glance, the recipe appears daunting. But it's not, you just need to proceed step by step. And remarkably, measurements are not very important. Let me restate the recipe as I see it.
First shopping: You need a leek or two (because it tastes better) and three different fish. I buy a half pound of each variety. I like to buy Tilapia because (a) it's cheap and (b) it's very firm. Then I'll buy scallops (bay are cheaper of course but I'll buy sea scallops and cut them into thin rounds). Finally, I choose a fish that is firm: Cod will do. Flounder? Absolutely not. And you'll need a red potato, a red pepper, some very small pasta (orzo is nice) and a baguette.
Next, the broth: Cut the leeks lengthwise in quarters (removing the green parts of course) and then cut into small quarter rounds. The onion should be diced. Start by slowly sauteing onions and leeks in olive oil (the recipe calls for 1/2 cup). You want them transparent but not brown. Next, add two cloves of garlic (smashed and then sliced). Then spices: I use thyme (fresh basil if you have it), maybe a few pistils of saffron, 2 bay leaves, and then two cups of diced tomatoes. If you have it, you can add an orange peel. If you don't, don't worry! Add 10 cups of water (yes, 10 cups!). Bring to a very soft boil. Now peel and cut the potato into large pieces, say, 8 and put into the stock. Finally, add a few sprigs of parsley.
Onward. Cut the baguette into thin (1/4 inch?) slices and put on a sheet pan. Put them in a 400 degree F oven for maybe 20 minutes. You want them dry but not really toasted --- just don't burn them. n.b. You can do this in advance.
Meanwhile, you can put a quarter to a half of the red pepper in boiling water. Also add two dried red peppers (if you like your rouille spicy, I do). I remove the stem end and de-seed them before adding to the water. Cook for a few minutes until soft. Drain and put into a blender jar. Add 2-4 cloves of garlic to the blender jar. Have fresh basil? Add some too. Also add a few sprigs of parsley.
Is the potato done? A knife should pierce it easily. If so, then fish out all the pieces and put into the blender jar. Next, add "sufficient" olive oil (the recipe calls for 4-6 tablespoons). I don't measure, but I try to make sure it'll grind. If it doesn't, I add more. The last step is to finish with stock (just a few tablespoons says the recipe), it should really make the rouille pourable but not too liquidy. Transfer the rouille to a serving bowl.
Almost done! The fish should be cut up into small portions, maybe 3/8 inch? Add about 1/2 cup of the pasta to the broth and then you add the Tilapia and scallops. Cod is last. Cook until the fish and pasta are done, about 10 minutes.
Now you're ready to serve. The "toasties" are ready to be placed in the bowl. Slather them with rouille. And then spoon the soup from the pot into the bowl. Serve with a crisp white, I happen to have fond memories of Picpoul de Pinet, but a California Sauvignon Blanc is just fine.