Step 1: Preheat your oven to 450°F (230°C) and place a large rimmed baking sheet inside to heat up.
Step 2: Place the potato chunks in a large pot and cover with cold water by at least an inch. Add a handful of salt and the baking soda.
Step 3: Bring to a boil over high heat, then reduce to a simmer. Cook for about 10 minutes, or until a knife meets little resistance on the exterior but the potato is still firm in the middle.
Step 4: Drain the potatoes and let them sit in the pot for 1 minute to dry. Shake the pot vigorously until a thick layer of mashed potato-like paste builds up on the edges of the chunks.
Step 5: In a small saucepan, heat the oil or fat with the rosemary, garlic, and pepper over medium heat until the garlic starts to turn golden. Strain the oil over the potatoes (reserve the garlic/herbs for later).
Step 6: Toss the potatoes in the oil until thoroughly coated. Carefully spread them onto the preheated baking sheet in a single layer.
Step 7: Roast for 45-50 minutes, turning with a spatula every 15 minutes, until deep golden brown and incredibly crispy. Toss with the reserved garlic and rosemary in the last 5 minutes of roasting.
Step 8: Season with extra salt if needed and serve immediately.