FOR THE CHICKEN:
2 ½ teaspoons kosher salt, plus more as needed
1 ½ teaspoons ground coriander
1 teaspoon finely grated lemon zest
¾ teaspoon finely grated fresh ginger
½ teaspoon black pepper
1 large garlic clove, grated or minced
1 (3 1/2- to 4 1/2-pound) chicken, patted dry
3 thyme sprigs, plus more for garnish
1 medium red onion, halved and sliced 1/4-inch thick
Extra-virgin olive oil, for drizzling
FOR THE RHUBARB:
Kosher salt, as needed
1 pound rhubarb, sliced crosswise 1/2-inch thick
½ cup granulated sugar
1 (2-inch) piece of fresh ginger, peeled and thinly sliced
½ teaspoon coriander seeds
3 thyme sprigs
In a small bowl, combine salt, ground coriander, lemon zest, ginger, pepper and garlic. Rub it all over chicken, including inside the cavity and under the skin. Stuff thyme sprigs into cavity.
Place chicken on a rimmed baking sheet, and marinate, uncovered, in the refrigerator for at least 1 hour or preferably overnight.
Prepare the rhubarb: Bring a small pot of salted water to a boil. Thinly slice the lemon into 1/8-inch rounds. Remove any seeds. Quarter lemon rounds into triangles. Blanch lemon pieces in boiling water for 2 minutes, then drain.
In a 9-by-9-inch baking dish or a pie pan, toss together blanched lemons, rhubarb, sugar, ginger, coriander seeds, a pinch of salt and thyme sprigs. Let fruit macerate at room temperature for 30 minutes to 1 hour. (You can do this while the chicken is marinating.)
When ready to roast, arrange an oven rack in the center of oven and another one below. Heat oven to 425 degrees. Toss onions in medium mixing bowl with a drizzle of olive oil and a pinch of salt, and set aside.
Generously drizzle olive oil over chicken. Roast chicken on the center rack and rhubarb below for 20 to 25 minutes, until rhubarb is glazed and syrupy.
Remove rhubarb and lower oven temperature to 400 degrees. Brush or spoon some of the rhubarb syrup all over chicken. Scatter red onion pieces around chicken on baking sheet, then continue to roast until skin is golden and cooked through and onions are browned, about 25 to 35 minutes longer.
Let chicken rest for 10 minutes. Carve and serve with onions, rhubarb and more thyme for garnish.