Season the chicken breasts with salt and pepper on both sides. Heat the oil in a large pan over medium-high heat. When hot, add the chicken and fry for 4 to 6 minutes per side until golden brown. Remove and set aside.
Reduce heat to medium. Dice the onions and add to the pan. Sauté for 3 to 5 minutes until softened.
Mince the garlic and add to the pan with the thyme. Cook for 1 minute, stirring frequently.
Add the wine to the pan, stirring to loosen any browned bits from the bottom. Add the tomatoes and tomato paste, and stir until combined.
Return the chicken to the pan. Cover with a lid and simmer over medium heat for 25 minutes, until the chicken reaches an internal temperature of 74°C (165°F).
Remove from heat. Add lemon juice to taste and season the sauce with salt and pepper as needed.
If desired, simmer uncovered for 5 to 10 minutes to reduce and thicken the sauce.