Preheat an oven to 400 degrees F (200 degrees C). Spoon 2 tablespoons olive oil over the cut sides of the garlic head; wrap tightly in aluminum foil. Roast in the preheated oven until the garlic is soft, about 30 minutes; set aside until cool enough to handle.
Squeeze the roasted garlic cloves into a saucepan; pour the chicken broth into the saucepan and use a fork to mash the garlic into the broth. Add onion. Cook the broth over medium-high heat until the liquid is reduced in volume by half, about 5 minutes. Stir the cream into the broth. Add the mussels; cover the pot and allow the mussels to simmer in the mixture just until they begin to open, about 5 minutes. Discard any mussels which do not open.
In a separate saucepan, melt the butter with 1 tablespoon olive oil in a large skillet over medium-high heat. Season the scallops with salt and pepper; sear in the hot butter and oil mixture until opaque, about 3 minutes per side. Remove from heat immediately.
Add the scallops to the mussels and cream sauce mixture. Stir in the lemon juice and spoon immediately onto 2 plates.