Preheat oven to 375°.
Drain tomatoes, reserving 1 1/2 teaspoons oil; chop tomatoes. Combine tomato, reserved oil, basil, and next 5 ingredients in a small bowl. Set aside.
Remove and discard giblets and neck from chicken. Rinse chicken with cold water; pat dry. Trim excess fat. Remove backbone from chicken, using kitchen shears. Flatten chicken, skin side up, with palm of hand. Starting at neck cavity, loosen skin from breast and drumsticks by inserting fingers, gently pushing between skin and meat. Stuff tomato mixture under skin of chicken.
Place onion in center of a jelly roll pan lined with foil. Place chicken, skin side up, over onion.
Bake at 375° for 50 minutes or until a thermometer registers 170°. Let stand 10 minutes. Remove and discard skin. Cut chicken into serving pieces; sprinkle with tomato mixture.