It is a food combination. According to the query of life knowledge website, "flatbread soup with mustard greens and green onions" indicates a food combination, in which "flatbread soup" is a traditional noodle soup, and "mustard greens" and "green onions" are the ingredients therein, therefore, this expression refers to a food combination of flatbread, mustard greens and green onions. The expression "flatbread soup" is a traditional noodle soup in which "mustard greens" and "green onions" are the ingredients, so the expression refers to a soup or cooking style consisting of flatbreads with mustard greens and green onions.