Step 1 Heat oil in a large heavy-based saucepan or flameproof casserole dish over medium-high heat (see note). Add onion. Cook, stirring, for 5 minutes. Add paste, tomato, stock and 1 1/2 cups cold water. Cover. Bring to the boil.
Step 2 Add penne. Bring to the boil. Reduce heat to medium. Simmer, uncovered, stirring occasionally, for 15 minutes or until pasta is tender and sauce has almost thickened, adding peas in the last 2 minutes of cooking. Season with salt and pepper. Stir in cream, mustard and half the chicken. Remove from heat.
Step 3 Stir in spinach. Serve topped with remaining chicken and parsley.