Preheat the oven to 350F.

Line a baking sheet with parchment paper.

Rinse the chicken, pat dry, and season on both sides with salt and pepper.

Wash the potatoes and halve them.

Peel and thinly slice the garlic cloves.

Peel and chop the onion.

Toss the potatoes, garlic, and onions with 4T of the olive oil.

Season with salt & pepper.

Transfer to the prepared baking sheet

Cut the rosemary sprigs into small pieces

Cut one and a half of the lemons into thin slices. Discard any pits.

Squeeze the juice out of the remaining half.

Make about 4 crosswise incisions in each chicken thigh, cutting into, but not all the way through the meat.

Tuck the lemon slices and rosemary into the incisions.

Lay the chicken over the potatoes.

Whisk together 2 tablespoons of olive oil and the lemon juice.

Pour the mixture over the chicken.

Tuck any extra lemon pieces into the potatoes.

Bake for 35–40 minutes.

Transfer to a serving platter.

Garnish with additional rosemary.

Serve with the rhubarb mayonnaise.

Cut the rhubarb stalk into pieces.

Boil with 2 tablespoons of water in a saucepan.

Use a wooden spoon to break up the rhubarb until it's mushy.

Strain out most of the liquid and let it cool completely.

In a tall container, add all the ingredients except the sugar.

Use an immersion blender to emulsify the mixture.

Taste and add sugar, salt, or more vinegar to taste.