Heat oil in a large nonstick skillet set over medium-high to high heat. Cook chicken, stirring often, for 5 minutes or until golden brown.
Add garlic; cook, stirring for 1 minute. Reduce the heat to medium.
Add tomato paste to pan; cook, stirring often for 2 minutes. Add soup, oregano, lemon zest and juice, soy sauce and pepper (optional); bring to a simmer, stirring often. Simmer, stirring often, for 4 minutes.
Meanwhile, cook pasta in unsalted water, according to package directions, adding spinach to pot in last minute of cooking. Drain, reserving 1 cup (250 mL) of the cooking liquid. Return pasta mixture to pan. Add soup mixture to hot pasta mixture. Stir in enough of the cooking liquid for sauce to evenly coat pasta.