Bring 2 quarts of salted water (1 tablespoon of salt) to a rolling boil. Add the Brussels sprouts and parboil them for 3 minutes or until just tender. (You could also steam them if you prefer.) They should be almost cooked all the way through (split one in half to test). Remove the sprouts with a slotted spoon to a bowl of ice water for a minute to shock the Brussels sprouts and help them retain their bright green color. Remove them from the ice water and cut the sprouts into halves. Heat 2-3 tablespoons of butter in a large sauté pan on medium heat. Add the chopped onions and cook until translucent, about 4-5 minutes. Add 2-3 tablespoons more of butter and the Brussels sprouts halves. Increase the heat to medium-high and cook for several more minutes. Salt and pepper to taste, while the Brussels sprouts are cooking. Do not overcook! Overcooked Brussels sprouts are bitter and are the main reason why some people don't like them. Remove the pan from the heat; stir in the lemon juice and half of the toasted almonds. Add salt and pepper to taste. Place in serving dish and garnish with the rest of the toasted almonds.