Make the ceviche: In a medium bowl, combine onion, juices, jalapeño, and salt.
Remove the tough muscles from the Side Dish of the scallops. Then halve the scallops horizontally. In a saucepan, bring 4 quarts of salted water to a simmer. Add scallops and poach until just cooked through (scallops should be opaque with centers slightly pink), about 1 minute. Drain scallops and gently toss with marinade. Marinate scallops, covered and chilled, 3 hours.
Make the pasta cakes: Cook pasta according to package directions. Reserve 1-cup cooking water, then drain pasta in a colander. Rinse briefly under cold running water to stop cooking (don't cool pasta) and drain well. Toss pasta with extra-virgin olive oil.
In a 10-inch nonstick skillet, heat 1/4 inch oil over medium heat until hot but not smoking. Form cakes by dropping a few strands of loosely squiggled pasta into a 1-tablespoon measure to fill it and inverting tablespoon into oil, flattening cake slightly if necessary. Cook cakes, 4 at a time, until crisp, 45 seconds to 1 minute per side, and drain on paper towels. Season cakes with salt. (If strands in bowl become too sticky to handle, stir in a little reserved pasta water, 1 teaspoon at a time. You need some starch from pasta to hold cake together, but you don't want strands to become wet.)
Make the salsa: Soak onion in cold water to cover for 20 minutes, then drain and rinse well. Stir together onion, cilantro, tomatillos, tomato, jalapeño, and salt and chill, covered, until ready to use.
To assemble: Lift scallops out of marinade and put 1 on top of each pasta cake. Top with salsa and serve immediately.