Make ceviche: Combine onion, juices, jalapeño, and salt in a bowl.
Poach scallops in 4 quarts of simmering salted water, stirring occasionally, 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 pasta cakes: Boil pasta in a large pot of boiling salted water until just cooked through, 1 to 2 minutes. Reserve 1 cup cooking water, then drain pasta in a colander. Rinse briefly under cold running water to stop cooking (dont cool pasta completely) and drain well. Toss pasta with extra-virgin olive oil.
Heat 1/4 inch olive or vegetable oil in a 10-inch nonstick skillet over moderate 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. (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 dont want strands to become wet.) Cook cakes, 4 at a time, until crisp, 45 seconds to 1 minute per side, and drain on paper towels. Season cakes with salt.
Make salsa: Soak onion in cold water to cover 20 minutes, then drain and rinse well. Stir together onion, cilantro, tomatillos, tomato, jalapeño, and salt and chill, covered, until ready to use.
Assemble hors d’oeuvres: Lift scallops out of marinade and put 1 on top of each pasta cake. Top with salsa and serve immediately.
Cooks' notes: ·Pasta cakes may be made 1 day ahead and kept between sheets of wax paper in an airtight container at room temperature. ·Ceviche can be made 1 day ahead and chilled, covered. ·Salsa is best when made no more than 2 hours before serving.