Preheat oven to 350 degrees.
In a large zipper-topped plastic bag, combine salt, pepper, cayenne pepper and half of the pressed garlic; shake to combine ingredients. Add beef and shake well to coat.
Thread beef cubes onto skewers and place on a lightly oiled metal rack or trivet inside a 9- x 13-inch baking dish; cover with foil and bake for 45 minutes to 1 hour or until the desired level of doneness is reached.
Meanwhile, in a small saucepan over medium heat, combine onion, remaining garlic, blueberries, vinegar (if using) and lemon juice
Bring mixture to a low boil and cook for 10 minutes, stirring often, until berries are heated through and mixture has reduced to a compote.
Stir in basil then bring to a low boil and cook for 1 to 2 minutes.
Divide spinach equally on dinner plates; top with beef skewers and drizzle with a few tablespoons of blueberry/basil sauce.