You can order any entree and dessert that is on the menu. San Angel dinner menu offers 8 entrees and 4 desserts on the a la carte section of the menu.
If you are looking at the "Traditional Mexican Dinner - DDP Special" on the dinner menu that's their prix fixe offering that includes an appetizer (Tostadas de Tinga), entree (Tacos de Pescado) and dessert (Crema Bavaria). Normally the DDP doesn't include an appetizer, but several restaurants have prix fixe offerings so that people who have the regular DDP, but want an appetizer have an option. The San Angel prix fixe doesn't give any choices, but most of the other prix fixe menus have 2 or 3 choices for each course. An example is the lunch prix fixe menu at Le Cellier.