Is 1 avacodo, handful almonds, sunflower seeds, walnuts, spinach leaves, tomatoe and onion a healthy lunch?

Surprisingly fatty. Sounds good on the surface, but would be rather caloric because of the amounts of healthy fats in the avocado and nuts and seeds. Would cut back on those ingredients, perhaps choosing one per day and switching them up.
May or may not be.. It depends on your body weight and activity level. If you are trying to lose weight, then you are ok with this type of eating for lunch. If you weight is normal or low, then you need to add some starch/carbs to this menu. But what you are eating for lunch is very healthy! so keep it up!