GDScript and C# NeoVim LSP Configuration

godot.lua

{
	"williamboman/mason-lspconfig.nvim",
	dependencies = {
		"williamboman/mason.nvim",
		"Hoffs/omnisharp-extended-lsp.nvim",
		"neovim/nvim-lspconfig",
	},
	config = function()
		require("mason").setup()
		require("mason-lspconfig").setup({
			automatic_installation = true,
			ensure_installed = {
				"omnisharp_mono",
			},
		})

		require("lspconfig").gdscript.setup({})

		require("mason-lspconfig").setup_handlers({
			-- The first entry (without a key) will be the default handler
			-- and will be called for each installed server that doesn't have
			-- a dedicated handler.
			function(server_name) -- default handler (optional)
				require("lspconfig")[server_name].setup({})
			end,
			-- Next, you can provide a dedicated handler for specific servers.
			-- For example, a handler override for the `rust_analyzer`:
			["omnisharp_mono"] = function()
				require("lspconfig").omnisharp.setup({
					handlers = {
						["textDocument/definition"] = require("omnisharp_extended").handler,
					},
					-- cmd = { "dotnet", "/path/to/omnisharp/OmniSharp.dll" },

					-- Enables support for reading code style, naming convention and analyzer
					-- settings from .editorconfig.
					enable_editorconfig_support = true,

					-- If true, MSBuild project system will only load projects for files that
					-- were opened in the editor. This setting is useful for big C# codebases
					-- and allows for faster initialization of code navigation features only
					-- for projects that are relevant to code that is being edited. With this
					-- setting enabled OmniSharp may load fewer projects and may thus display
					-- incomplete reference lists for symbols.
					enable_ms_build_load_projects_on_demand = false, -- default false

					-- Enables support for roslyn analyzers, code fixes and rulesets.
					enable_roslyn_analyzers = true, -- default false

					-- Specifies whether 'using' directives should be grouped and sorted during
					-- document formatting.
					organize_imports_on_format = true, -- default false

					-- Enables support for showing unimported types and unimported extension
					-- methods in completion lists. When committed, the appropriate using
					-- directive will be added at the top of the current file. This option can
					-- have a negative impact on initial completion responsiveness,
					-- particularly for the first few completion sessions after opening a
					-- solution.
					enable_import_completion = true, -- default false

					-- Specifies whether to include preview versions of the .NET SDK when
					-- determining which version to use for project loading.
					sdk_include_prereleases = true,

					-- Only run analyzers against open files when 'enableRoslynAnalyzers' is
					-- true
					analyze_open_documents_only = true, -- default false
				})
			end,
		})
	end,
}

The solution was not to use omnisharp-mono at all, and simply stick to omnisharp. I had to still supply a full path to the omnisharp executable in the config, though. Here's the commit: InfiniteRain/nvim@fcf5899

My setup now fully works with Godot's C#!