Slow-Braised Short Ribs with Spinach

2 tablespoons garlic salt
1 tablespoon sweet paprika
Kosher salt and freshly ground black pepper
1/2 cup canola oil
Two whole-plate, 3-bone short ribs (about 9 pounds total), trimmed of excess fat
1 ounce dried porcini mushrooms (3 cups)
3 cups boiling water
1/3 cup extra-virgin olive oil
1 very large white onion, diced
2 carrots, finely diced
4 celery ribs, finely diced
8 garlic cloves, crushed
1 pound white button mushrooms, stemmed and quartered
1 cup dry white wine
3 cups beef stock or low-sodium broth
8 ounces matzo farfel (4 cups), finely crushed (see Note)
4 cups baby spinach (4 ounces)
2 tablespoons chopped flat-leaf parsley
Coarse sea salt