Mix softened butter, minced garlic, chopped sage, rosemary, thyme, parsley, 1½ tsp salt, and ½ tsp pepper until combined.
Pat turkey dry. Loosen skin from breast with a knife, then slide two-thirds of herb butter under the skin.
Spread remaining butter over the turkey skin and underside. Season skin with ½ tsp salt and pepper.
Place halved onion and garlic head in slow cooker. Set turkey skin-side up on top. Scatter rosemary and thyme sprigs over.
Cover and cook on low for 6 hours, until internal temperature reaches 75°C (165°F).
Transfer turkey to a baking tray, cover loosely with foil, and rest 20 minutes.
Remove herb sprigs, drizzle turkey with oil, and broil 5-10 minutes until skin is deep golden and crispy.
Strain slow cooker juices into a bowl, pressing onion and garlic to extract liquid. Add broth to reach 2½-3 cups if needed.
Let liquid settle, then spoon off ¼ cup fat into a saucepan. Heat over medium, whisk in flour, and cook 1 minute.
Slowly whisk in strained juices, simmer 3-5 minutes until thickened. Season with soy sauce, pepper, and salt to taste.