Place potatoes in a pot and cover with cold water. Put a lid on the pot and bring the water up to a boil. Salt water and cook potatoes until tender, 15 minutes.
Bring 1/2-inch water to a simmer in a large nonstick skillet. Zest a lemon and reserve. Cut lemon and season the fish the some lemon juice and salt and add to the simmering water. Cover pan and cook fish 8 minutes.
Remove and flake fish into a bowl. Discard water and wipe out pan, returning it to heat over medium flame and add 1/4 cup vegetable oil. To the fish, add reserved lemon zest, seafood seasoning, celery, onion, thyme, parsley, salt, and pepper to fish. Add egg and 1 cup of the bread crumbs. Form 8 fish patties and toss in remaining bread crumbs to set the patties.
Add 1/4 cup vegetable oil to the cleaned out pan and heat over medium flame.
Cook patties in the oil for 4 to 5 minutes on each side until golden and reserve under foil or in warm oven. Return skillet to heat again over medium flame and melt in 2 tablespoons of butter then add the red bell peppers and shallots and cook 2 to 3 minutes. Add in flour and cook a minute more. Whisk in broth, thicken, and season with salt and pepper to make savory red pepper gravy.
Drain cooked potatoes and return to hot pot. Add in remaining butter, garlic and 1/4 cup cream or half & half. Mash and season the potatoes with salt and pepper. Fold in spinach leaves until they all wilt into potatoes.
To serve, pile spinach mashers on plates and top with patties and gravy.