1 In a small saucepan, combine the potatoes, garlic, cold water, and 1/4 teaspoon of the salt and bring to a boil over moderate heat.
Cook, uncovered, for 1 0 minutes or until the potatoes are tender.
Drain, reserving 1/4 cup of the cooking liquid.
Mash the potatoes with the garlic; you should have about 1 1/3 cups.
2 In a large bowl, combine the yeast, sugar, and lukewarm water and let stand for 5 minutes or until foamy.
Stir in the mashed potatoes, reserved cooking liquid, remaining 2 teaspoons of salt, the milk, and oil.
With a wooden spoon or an electric mixer set on low speed, beat in 3 cups of the flour, then gradually add enough of the remaining flour to make a soft dough.
Divide the dough in half and transfer to a lightly floured surface.
3 Knead the spinach into half of the dough until smooth, elastic, and well combined-about 10 minutes by hand or 6 minutes with an electric mixer.
Transfer to a lightly greased large bowl, turn to coat with the grease, and cover loosely with plastic food wrap.
Let rise in a warm, draft-free place for 45 minutes or until doubled in bulk.
Repeat with the tomato paste and remaining half of the dough.
4 Lightly grease a baking sheet.
Punch down the spinach dough, place on a lightly floured surface, and pat the dough out to a 1 2- by 14-inch rectangle.
Place the tomato dough on top and pat it out to a 12-by 1 3-inch rectangle.
Roll up the 2 layers, jelly-roll style, to form a loaf 6 by 1 3 inches.
Place on the baking sheet, seam side down, tuck under the ends, cover loosely, and let rise for 30 minutes or until light and puffy.
5 Preheat the oven to 400° F.
Brush the top of the loaf with the beaten egg.
Bake for 50 minutes or until the loaf is golden brown and crusty and sounds hollow when tapped on the bottom.
Let cool on a wire rack