Oven baked piroshki with jam are famous for being tricky to make, because often times the preserve filling may try to escape the pie. All of that is easy to avoid if you are making a good yeast dough that would hold together, and very important, if jam is thick enough. Even if jam, whether it is homemade or store-bought looks super thick, it would melt in the oven during the baking process and thus try to escape. It almost ALWAYS needs to be thickened. The starch or flour are a good way to thicken it as during the baking it would be absorbing the jam’s liquid and nothing would escape piroshki. Another important step is closing the pirozhok so that there is no way the filling gets out. This braid style that I love using is super easy and almost foolproof, especially if the dough is mixed well and was given enough time to rest.