Heat oil in 12-inch heavy skillet on medium-low heat until hot. Add onion, garlic, cayenne, and bell pepper; cook uncovered until tender, about 10 minutes. Add lemon juice, brown sugar, zest, salt and pepper; heat one minute.
While preparing onion/pepper mixture, bring a large pot of water to a boil; salt to taste. Cook pasta until tender, 8-10 minutes. About one minute before pasta is cooked, add spinach. Drain pasta and spinach and place in a large, warm serving bowl. Add onion/pepper mixture and toss to coat. Taste and adjust seasonings. Add previously cooked and warmed scallops to pasta and sprinkle with feta cheese.
