1. Place potatoes in a large pot and add enough water to cover. Bring to a boil over medium-high heat and cook until tender, about 30 minutes. Drain and let cool slightly.

2. When potatoes are cool enough to handle, peel and pass through a ricer or food mill, discarding peels. Spread mashed potatoes out on a large baking sheet and season with salt; let cool.

3. Add egg to cooled potatoes and sprinkle with 3/4 cup flour. Gather mixture together and turn out onto work surface; knead, adding more flour as necessary to prevent sticking, until dough comes together. Do not knead too much or add too much flour, as gnocchi will become heavy.

4. Cut dough into three equal pieces. Lightly flour work surface and hands. Roll each piece into a 1/2-inch-thick rope; cut each rope into 1/2-inch pieces. Roll each piece lightly in the palms of your hands. Holding a fork with one hand at a 45-degree angle, use your thumb to press lightly on one piece of dough, starting high on the inside tines of the fork. Roll dough down the whole length of the tines to create ridged gnocchi with an indentation where your thumb was. Repeat process with remaining pieces of dough. Dust gnocchi with flour and place on a large baking sheet.

5. Preheat oven to 375 degrees.

6. Bring a large pot of salted water to a boil over medium-high heat. Add gnocchi to boiling water, cooking in batches if necessary. Once gnocchi rise to the surface, continue cooking about 2 minutes more. Drain.

7. Transfer gnocchi to a small baking dish and add marinara sauce; sprinkle with cheeses. Transfer baking dish to oven; bake until warmed through, about 6 minutes. Serve.