Preheat oven to 425°F with a rack in the center.
To make the. marinade, lace the olive oil, garlic, herbs, salt, and pepper into a mortar. Zest the lemon directly into the mortar, and add the lemon juice.
Bash the mixture with the pestle, crushing the garlic against the sides and bottom, to form a rough, chunky marinade.
Dry the chicken well with paper towels and place it skin-side-up in a 12-inch cast iron skillet.
Pour the marinade overtop and use your hands to distribute it evenly over the chicken.
At this point you can roast right away or leave the chicken to marinate at room temperature for 30 minutes or in the fridge for up to 24 hours.
Roast for about 40 minutes depending on size, until the juices run clear when you insert a knife between a leg and thigh, and breast meat reads 150°F on an instant thermometer.
Let the chicken rest or at least 10 minutes before carving.
If you'd like to make gravy, set the skillet over medium heat while the chicken rests. Whisk the flour into the pan drippings.
When incorporated, pour in the broth and whisk until smooth.
Bring to a boil, them simmer for five minutes until thickened. (You can adjust the amount of broth to suit your preference.)
Whisk in the soy sauce and marmite and serve.