In a large bowl, arrange spinach and red onion slices and top with toasted pecans and crumbled blue cheese.
In a small saucepan, warm the garlic in the olive oil to allow its flavor to diffuse into the oil. Then add molasses, vinegar and pepper. Whisking constantly, bring almost to a boil, remove from heat and cool until just warm. Pour into a shakeable container, shake and pour over salad. Serve immediately.