Heat 2 teaspoons oil in a large, non-stick frying pan over a medium-high heat. Add chicken. Cook for 3-4 minutes each side or until browned and just cooked through. Transfer to a plate. Cover loosely with foil.
Heat remaining oil in same pan. Add garlic, tomatoes and mushrooms. Cook, stirring for about 3 minutes, or until mushrooms are tender and lightly browned.
Stir in evaporated milk and mustard. Bring to the boil. Reduce heat. Gently boil for about 5 minutes, stirring often, until thickened.
Add spinach and stir over heat until wilted.
Return chicken to pan with any juices from plate. Turn chicken to coat with sauce. Cook a further 1-2 minutes, until hot. Serve with boiled baby potatoes.

If small chicken breast fillets are unavailable, purchase two large chicken breast fillets (about 300g each) and cut horizontally in half to give 4 smaller fillets.