In a saucepan, put the onion cut in half, carrot chunks, celery stalk cut into pieces, bay leaf, parsley, and thyme sprigs. Remove the shells from the shrimp and put the shells in the saucepan as well. Fill with water about half way full and cook for 20 minutes after it starts to boil.
In a second larger soup pot, add olive oil and butter, fry the chopped carrots, onions, celery for 5 minutes, stirring occasionally.
Add garlic and fry for 2 more minutes. Pour in white wine, lemon juice and bring to a boil. Strain the broth in your first pan using a fine mesh strainer, pouring the liquid into the veggie pot. Discard the remaining onions, carrots, celery, herbs and shrimp shells in the strainer.
Add cubes of peeled potatoes plus more water/vegetable broth if necessary in order for them to be submerged. Boil until they are fork tender.
Then add shelled shrimp, mussels, clams if adding those and corn. Add salt and pepper. Boil until mussels open and shrimp is pink and no longer transluscent, about another 5-10 minutes.