Wash and dry sweet potato. Leaving the skin on, slice sweet potato into long French fry thick slices, about 5 inches long and no more than ½-inch wide.
Heat beef tallow in a large cast iron skillet over medium heat until hot. Lay sweet potato slices in the skillet in a single layer, avoiding overcrowding by cooking in two batches if needed.
Fry sweet potatoes until golden, crispy, and cooked through, flipping halfway through, for about 10 minutes. Keep warm.
Make the bone broth gravy:
In a small saucepan, bring broth, onion and garlic to a boil over medium-high heat. Reduce heat to medium-low and fit lid loosely on saucepan to allow steam to escape.
Simmer for 25 to 30 minutes until onion and garlic are tender. Transfer to a high-powered blender and blend until smooth.
Transfer gravy back to the saucepan and bring to a boil again over medium-high heat. Reduce heat to medium-low, leave uncovered, and reduce gravy for about 15 minutes until it's thick enough to coat a spoon. Keep warm.
Roast the garlic:
Preheat oven to 400°F. Line baking sheet with parchment paper. Toss garlic cloves with olive oil and sea salt and spread evenly on prepared sheet.
Roast for 15 to 20 minutes until golden and tender. Set aside.
Assemble the Poutine:
Assemble poutine by layering fries in serving dish with gravy drizzled on top and garnished with roasted garlic. Serve immediately.