Heat a large frypan over medium heat and drizzle the olive oil into the hot frypan. Cook and stir garlic just until fragrant; about 1 minute smashing them into the oil with a spoon as you stir.
Sprinkle the chicken breasts with salt and pepper then brown them in the hot oil until the surface is golden and the meat is no longer pink inside; 5 to 8 minutes per side. Remove the chicken breasts to a platter and keep warm.
Heat 1 tablespoon of olive oil in the frypan over medium heat and scrape up any browned bits of flavour from the bottom of the frypan. Stir in 2 more cloves of garlic, the green, red and yellow capsicums, red onion and asparagus then cook and stir until the vegetables are brightly coloured and softened; about 10 minutes.
Toss the spinach leaves with the hot vegetables to just wilt the spinach. Divide the cooked vegetables between 4 serving plates and top each plate with a chicken breast.
Stir the balsamic vinegar and brown sugar into the frypan over medium-low heat stirring until the vinegar comes to a full boil and the sugar has dissolved.
Cook the mixture until reduced and thickened; 3 to 5 minutes. Spoon the balsamic sauce over each chicken breast to serve.